PDF Glossary

Also known as: AES-256, AES 256-bit, 256-bit encryption

AES-256 is a strong encryption method that scrambles data with a 256-bit key. It is the strongest encryption the PDF format supports, available in PDF 2.0 and in Acrobat 9 and later. When you password-protect a PDF with AES-256, its text and images stay unreadable until someone enters the right password. Guessing the key by brute force is not practical, so the password itself is the weak point.

Also known as: accessible PDF forms, tagged forms, form fields

Accessible forms are fillable PDF forms that people using screen readers or only a keyboard can complete. Each field has a clear label stored as a tooltip, fields follow a logical tab order, and the form is tagged. If you publish forms for the public, rules such as WCAG, Section 508 and PDF/UA expect this. Good labels also mean fewer wrong or missing answers from every user.

Also known as: Acrobat form, interactive PDF form, native PDF form

An AcroForm is the standard type of fillable form built into the PDF format. Its text boxes, checkboxes, dropdowns and buttons are stored inside the file, and each one appears on the page as a widget annotation. Almost every PDF reader can fill, save and print an AcroForm. If a form's fields are clickable in your browser or phone viewer, it is very likely an AcroForm.

Also known as: ActualText

ActualText is a hidden value in a tagged PDF that gives the real text behind a piece of content. It is useful when what is drawn on the page does not match the letters, such as an 'fi' ligature, decorative lettering or a picture of a word. Screen readers read the ActualText aloud, and search and copy use it too, so the words come out right.

Also known as: AES, AES-256, AES-128

The Advanced Encryption Standard (AES) is a widely used encryption method approved by NIST. It scrambles data in 128-bit blocks using a 128-bit or 256-bit key. PDFs have used AES-128 since PDF 1.6 and AES-256 since Acrobat 9. PDF 2.0 made AES-256 part of the ISO standard. It replaced the older RC4 cipher, which is now considered broken. If you password-protect a PDF today, choose AES so the content stays private.

Also known as: alt text, alternative text, alt attribute

Alternate text, or alt text, is a short written description of an image, chart or other visual element. Screen readers read it aloud to people who cannot see the picture. In a tagged PDF it is stored in the element's Alt entry. Accessibility rules such as WCAG, Section 508 and PDF/UA require alt text on meaningful images, while purely decorative images should be marked so they are skipped.

Also known as: PDF annotation, annot

An annotation is anything added on top of a PDF page without changing the page itself. Notes, highlights, shapes, stamps, links and form fields are all annotations. Each is stored in the page's /Annots list with its own position and look. Because they sit in a separate layer, you can add, hide or remove annotations later and the original text and layout stay the same.

Also known as: AP stream, normal appearance, /AP

An appearance stream is a small set of drawing instructions that tells a viewer exactly how an annotation or form field should look. It is stored under the annotation's /AP entry. If an appearance stream is missing, each viewer draws the item its own way, so a filled field or stamp may look different or vanish when printed. Flattening a PDF uses these streams.

Also known as: PDF Array, Array

An array object is an ordered list of values inside a PDF file, written between square brackets. For example, [0 0 595 842] is the page box of an A4 page in points. Arrays hold coordinates, page sizes, dash patterns and lists of references to other objects. You only meet them when you open a PDF in a text editor or work with it in code.

Also known as: art box

The ArtBox is an optional page box that marks the area of a PDF page holding the meaningful content, as the designer intended. If it is not set, it is the same as the CropBox. It matters mostly when the page is placed as artwork in another layout, such as an advert in a magazine, where design tools use it to decide what part of the page to import.

Also known as: article, threaded article, reading thread

An article thread is a reading path that links boxes of text across columns and pages in a set order. When a reader follows the thread, the viewer jumps and zooms from one box to the next. It helps with magazine or newspaper PDFs, where a story continues in another column or on a later page. Most mobile and browser viewers ignore article threads.

Also known as: artifacts, PDF artifact

An artifact is content on a PDF page that is only there for decoration or layout, such as page numbers, running headers, background images or divider lines. In a tagged PDF, marking these as artifacts tells screen readers to skip them, so users hear the real content without repeated clutter. PDF/UA, the accessibility standard for PDFs, requires this marking.

Also known as: ASCIIHexDecode, ASCII Hex, ASCIIHex

ASCIIHexDecode is a PDF filter that stores binary data as plain text, writing each byte as two hexadecimal digits. It does not compress anything. In fact it doubles the size of the data. It was meant for old channels that could only carry plain text. You will rarely see it today, and removing it from a file makes the file smaller without losing anything.

Also known as: ASCII85, ASCII85Decode, Base85

ASCII85 is a way to write binary data as plain printable text, turning every four bytes into five characters. In a PDF, the ASCII85Decode filter turns it back into the original bytes. It is not compression and makes data about 25 percent larger. It is uncommon now, and removing it from a file's streams reduces file size without changing what you see.

Also known as: ADF, document feeder

An Automatic Document Feeder (ADF) is the tray on a scanner or copier that pulls a stack of pages through one at a time. It lets you scan many pages, often both sides, into a single PDF without lifting the lid for each sheet. The trade-off is that pages can go in slightly crooked or two at once, so check for skewed or missing pages afterwards.

Also known as: standard 14 fonts, base 14 fonts, standard fonts

The base-14 fonts are 14 standard fonts that every PDF viewer must provide: Helvetica, Times and Courier in four styles each, plus Symbol and Zapf Dingbats. A PDF can use them without embedding the font file, which keeps the file small. The catch is that each viewer supplies its own version, so text can look slightly different from one device to another.

Also known as: Bates stamping, Bates labeling, Bates numbers

Bates numbering is a way of giving every page in a set of documents its own unique number, in order. A typical Bates number looks like ABC000001, which is a prefix such as a case or client code followed by a zero-padded page number. The next page is ABC000002, and so on. Lawyers use Bates stamps in legal discovery so anyone can point to one exact page.

Also known as: thresholding, black-and-white conversion

Binarization turns a color or grayscale scan into pure black and white. Each pixel is compared with a brightness threshold and becomes either black text or white background. Adaptive methods change the threshold across the page to cope with shadows or uneven light. For scanned PDFs this gives sharper letters, better OCR results and much smaller files, though photos and faint marks can be lost.

Also known as: bits per pixel, color depth, bits per component

Bit depth is how many bits are used to store the color of each pixel in an image. Common values are 1-bit for black and white, 8-bit for grayscale and 24-bit for full color. More bits mean more colors and bigger files. Turning a color scan of a text page into grayscale or black and white is one of the easiest ways to shrink a PDF.

Also known as: bleed area, full bleed

Bleed is extra artwork that runs past the edge where a printed page will be cut, usually 3 mm (about 0.125 inch). Paper shifts a little during trimming, so without bleed you can get thin white lines along the edges. If you send a PDF with colors or images reaching the page edge to a printer, add bleed and make sure it is set in the file's BleedBox.

Also known as: bleed box

The BleedBox is a page box in a PDF that marks the page area plus its bleed, the extra margin that will be cut off after printing. If it is not set, it is the same as the CropBox. Print shops use it to know how far artwork extends past the final trim line, so a print-ready PDF with edge-to-edge color should include a BleedBox larger than the TrimBox.

Also known as: Blend Modes, Multiply, Screen

A blend mode is a rule for how a transparent object's colors mix with whatever is underneath it. Multiply darkens, Screen lightens and Overlay adds contrast. PDF supports 16 blend modes. They matter mainly for printing, because some blends behave unexpectedly with spot colors or overprint, and can change appearance when a file is flattened or converted to another color space.

Also known as: outline, document outline, PDF bookmarks

A bookmark is a clickable entry in a PDF viewer's side panel that jumps to a page or section. All the bookmarks together are called the document outline, and they can be nested like a table of contents. For long PDFs such as manuals or reports, bookmarks let readers reach a chapter in one tap instead of scrolling. Splitting or merging files can keep or drop them.

Also known as: COS, COS syntax

The Carousel Object System (COS) is the basic building layer of every PDF file, named after Adobe's early 'Carousel' project. It defines eight simple object types: Boolean, number, string, name, array, dictionary, stream and null, plus references between objects. Pages, fonts, forms and everything else in a PDF are made from these objects. Developers working on PDF libraries deal with it directly.

Also known as: CCITT Group 4, CCITTFaxDecode, G4 fax, Group 4

CCITT Group 4 is a lossless compression method for black-and-white images, first made for fax machines. In a PDF it is decoded by the CCITTFaxDecode filter. It stores where each line changes from black to white compared with the line above, which works very well for scanned text. A black-and-white scanned page saved this way is often only a few tens of kilobytes.

Also known as: CA, certificate authorities

A Certificate Authority (CA) is a trusted organization that checks people's or companies' identities and issues digital certificates to them. A certificate links a person to the key used to sign documents. When you open a digitally signed PDF, your viewer checks whether the signer's certificate comes from a CA it trusts. If it does not, the signature shows as unknown or not trusted.

Also known as: CRL, Revocation List

A Certificate Revocation List (CRL) is a signed list, published by a Certificate Authority, of certificates it has cancelled before their expiry date. When you check a digital signature in a PDF, the software can look at the CRL to confirm the signer's certificate was still valid at signing time. Storing the CRL inside the PDF lets the signature be checked years later.

Also known as: DocMDP, Certifying Signature, MDP

A certification signature is the first digital signature on a PDF, added by its author to vouch for the document. It sets DocMDP permissions that state what later changes are allowed: no changes, only form filling, or form filling plus comments. Any change outside those rules breaks the signature. Recipients can then trust that the file came from the certifier and was not tampered with.

Also known as: encoding, font encoding

Character encoding is the table that links the codes stored in a PDF's text to actual letters or glyphs in a font. WinAnsiEncoding is a common example. The encoding decides which shape is drawn and what text you get when you search or copy. A missing or custom encoding is a common reason text looks fine on screen but pastes out as random symbols.

Also known as: check box, tick box

A checkbox is a form field that is either ticked or not, for a single yes or no answer. In a PDF, each state has its own stored appearance, and the unticked state is always called Off. Checkboxes are easy to fill on any device, export cleanly as data and can be flattened so the tick becomes a permanent part of the page.

Also known as: chroma subsampling, 4:2:0, YCbCr subsampling

Chroma subsampling is a trick in JPEG compression that stores color at a lower resolution than brightness, because the eye notices color detail less. The common 4:2:0 scheme keeps only a quarter of the color samples. For photos in a PDF this cuts file size with little visible loss. It works badly on sharp colored text or lines, which can show smudged edges.

Also known as: CID font, CID-keyed font, composite font

A CID-keyed font is a font that finds each glyph by a number, called a Character Identifier, instead of by name. This design suits fonts with thousands of characters, such as Chinese, Japanese and Korean. In a PDF these fonts appear as Type 0 fonts with a CMap that converts text codes to CIDs. They decide whether non-Latin text displays and copies correctly.

Also known as: CMap, character map

A CMap is a table that tells a PDF reader how to turn the bytes in a page's text into character identifiers for a large font. It lets one font cover thousands of characters using one-byte or two-byte codes. CMaps are needed for Chinese, Japanese, Korean and other large scripts. A broken or missing CMap is a common cause of garbled or missing characters.

Also known as: CAdES

CAdES (CMS Advanced Electronic Signatures) is a European standard from ETSI for digital signatures. It builds on the CMS/PKCS#7 signature format and adds extras such as a reference to the signing certificate, timestamps and validation data. CAdES can sign any file. The PDF version of these rules is called PAdES. Both support advanced electronic signatures as defined by the EU eIDAS regulation.

Also known as: CMYK, process color, four-color process

CMYK is the color system used in printing. It builds colors from four inks: cyan, magenta, yellow and key, which means black. Print shops usually want PDFs in CMYK so the printed result matches the design. CMYK can show fewer colors than the RGB used on screens, so bright screen colors, such as vivid greens or blues, may look duller on paper.

Also known as: gamut, colour gamut

A color gamut is the full range of colors a screen, printer or color space can show. Screens using RGB can show more vivid colors than a CMYK printing press. Colors outside the printer's gamut shift when a PDF is converted for print, often looking duller. Soft-proofing the PDF on screen before printing shows you which colors will change.

Also known as: colour management, color-managed workflow

Color management is the process of keeping colors consistent across cameras, screens and printers. It uses ICC profiles, small files that describe how each device shows color, and converts between them. In PDF work it helps the printed result match what you saw on screen. Without it, the same PDF can look noticeably different on two monitors or two printers.

Also known as: separations, color separation

Color separations split a color document into one layer per ink, such as cyan, magenta, yellow, black and any spot colors. A printing press prints each layer from its own plate to build the final image. Previewing separations in a PDF lets you check that each color lands on the right ink before printing, which avoids wasted plates and press time.

Also known as: Comb Text Field, Comb

A comb field is a PDF text field split into a fixed number of equal boxes, with one character per box. It is common for phone numbers, dates, postcodes and ID numbers on official forms. To work, the field needs a maximum length set, which also decides the number of boxes. Typed characters then line up neatly with the boxes printed on the form.

Also known as: list box, dropdown, choice field, select field

A combo box is a dropdown field in a PDF form that lets the user pick one option from a list. It can also allow typing a custom answer if the form author enables that. A list box is similar but shows several options at once and can allow more than one choice. Both keep answers consistent and easier to check and export.

Also known as: review comment, PDF comment, reply

A comment is a note left on a PDF during review, attached to a highlight, sticky note or other markup. Each comment stores its text, the author's name and the date, and others can reply to it in a thread. Comments let a team discuss exact passages inside the file itself. You can remove them all before sending the final version.

Also known as: Compare PDF, PDF Diff, Document Comparison

Document comparison checks two versions of a PDF and shows what changed between them, such as added, deleted or edited text. Results are usually shown side by side or with the changes highlighted. It is useful for checking contract revisions, proofs or drafts, because you can see every edit at once instead of reading both files line by line.

Also known as: Type 0 Font, CID-keyed Font, Composite Font

A composite font, or Type 0 font, is a PDF font that uses multi-byte codes, so one font can hold thousands of glyphs. This is needed for Chinese, Japanese, Korean and full Unicode text. It points to one CIDFont, which holds the glyphs, and a CMap, which converts codes into glyph numbers. Simple PDF fonts, by contrast, are limited to 256 characters.

Also known as: page content stream, content streams

A content stream is the part of a PDF that holds the drawing instructions for a page: where to place text, lines, shapes and images. Each page points to one or more content streams, which a viewer reads in order to draw the page. When a tool edits, extracts or redacts text, it is reading or rewriting these instructions behind the scenes.

Also known as: PDF coordinates, page coordinates

The PDF coordinate system measures positions on a page in points, where one point is 1/72 inch. The starting point (0, 0) is the bottom-left corner, with x growing to the right and y growing upward. Screens usually start at the top-left instead. This matters when you place a signature, stamp or watermark at an exact spot, because heights are measured from the bottom.

Also known as: CreationDate, ModDate, PDF date metadata

The creation and modification dates are two metadata fields that record when a PDF was first made and when it was last changed. They are stored as CreationDate and ModDate, with a time zone offset. They help with version tracking, records rules and legal reviews. Because converting or editing a file can reset or remove them, investigators do not treat them as proof by themselves.

Also known as: crop box

The CropBox is the rectangle that sets which part of a PDF page is visible when it is shown or printed. If it is not set, it matches the MediaBox, the full page. Cropping or trimming a PDF usually just changes the CropBox, so the margins are hidden rather than deleted. The hidden content is still in the file and can be restored.

Also known as: xref stream, compressed xref, cross-reference stream

A cross-reference stream is a compressed version of the cross-reference table, added in PDF 1.5. It stores the location of every object in binary form inside a stream, which makes files smaller. It is required when a file packs objects into object streams. Most modern and optimized PDFs use cross-reference streams, so you will rarely see a plain xref table in new files.

Also known as: xref, xref table, cross reference table

The cross-reference table, or xref table, is a list near the end of a PDF file that gives the exact position of every object in the file. A reader uses it to jump straight to any page, font or image without reading the whole file. When this table is damaged, a PDF often fails to open, and repair tools work by rebuilding it.

Also known as: CF, StmF, StrF

A crypt filter is a setting inside an encrypted PDF that says how a specific part of the file is encrypted. It lets one PDF mix methods, for example encrypting the page content with AES while leaving the metadata readable so search tools can still index the file. Crypt filters were added alongside AES support and give finer control over what is protected.

Also known as: hash, message digest, digest, SHA-256

A cryptographic hash, also called a message digest, is a short fixed-length fingerprint calculated from a file with an algorithm such as SHA-256. Changing even one byte of the file produces a completely different hash. Digital signatures in PDFs sign this fingerprint, not the whole file. When you open the PDF, the viewer recalculates the hash, so any later change is detected.

Also known as: PKCS#7, PKCS7, CMS

Cryptographic Message Syntax (CMS), first published as PKCS#7, is the standard format for packaging a digital signature. The package holds the signed hash of the document plus the signer's certificates. A signed PDF stores this package inside its signature field. When you open the file, your viewer unpacks it, checks the certificates and confirms the document has not changed since signing.

Also known as: custom document properties, custom properties

Custom metadata is any extra information property added to a PDF beyond the standard fields such as Title and Author. It is stored in the document information dictionary or in XMP. Companies use it for things like an ID, a department code, a workflow status or a security label. Document systems can then sort or route files automatically, but custom fields may reveal internal details when shared.

Also known as: skew correction, straighten scan

Deskew means straightening a scanned page that went into the scanner at a slight angle, so the lines of text run level. Auto deskew detects the angle of the text and rotates the page by that amount for you. Even a tilt of a few degrees makes scans harder to read and lowers OCR accuracy, so deskewing is a standard first cleanup step for scanned PDFs.

Also known as: noise removal, speckle removal

Despeckle is a cleanup filter that removes small stray dots from a scanned page, such as dust, paper flecks and scanner noise. It deletes tiny isolated marks while keeping real letter strokes. On a scanned PDF this gives a cleaner page, fewer fake commas or periods in OCR results and a smaller file, because clean black-and-white images compress better.

Also known as: pdf dictionary, dictionary

A dictionary object is a set of named entries inside a PDF file, written between << and >>. Each name is paired with a value, for example << /Type /Page /MediaBox [0 0 595 842] >> describes an A4 page. Pages, fonts, annotations and the document catalog are all dictionaries. You only see them when you inspect a PDF's raw structure or work with it in code.

Also known as: DOI, digital object identifier

A Digital Object Identifier (DOI) is a permanent ID given to an article, report or dataset, such as 10.1000/182. It always points to the work's current location, even if the website address changes, so citations keep working. Academic PDFs often show the DOI on the first page and store it in their XMP metadata, which lets reference managers identify the paper automatically.

Also known as: DRM, PDF DRM, rights management

Digital rights management (DRM) is a system that controls who can open, print, copy or forward a PDF, usually through a licence server. It goes further than normal PDF passwords and permissions. The publisher can set an expiry date or revoke access after the file is sent. The downside is that readers usually need specific software or an account to open the document.

Also known as: cryptographic signature

A digital signature is a secure electronic signature that proves who signed a document and that it has not changed since. It uses a certificate and a private key. The software calculates a fingerprint of the file and signs that fingerprint. When you open a signed PDF, the viewer checks the signature, and any edit made after signing shows the signature as invalid.

Also known as: DCTDecode, JPEG, DCT

DCT, the Discrete Cosine Transform, is the math behind standard JPEG compression. In a PDF, JPEG images use the DCTDecode filter. The image is split into 8x8 pixel blocks, and fine detail in each block is thrown away to save space. This works well for photos, but it is lossy, so text and sharp lines get blurry edges and quality drops each time it is re-saved.

Also known as: distiller, PostScript distilling

Distilling is the process of turning a PostScript or EPS file into a PDF. The name comes from Adobe Acrobat Distiller. The distiller embeds fonts, compresses or downsamples images and applies a settings profile. Print shops still use the term, because the chosen job options decide the image resolution, compression and PDF/X compliance, and so the quality and size of the final file.

Also known as: PDF assembly, collate, document collation

Document assembly is putting together a finished PDF from several files or pages, for example a cover letter, contract and appendices merged in the right order. It often includes reordering, inserting or deleting pages. The result is one complete file that is easier to send, print and file. In PDF permissions, document assembly is also the right to insert, delete or rotate pages.

Also known as: author, PDF author metadata

The Author field is a piece of PDF metadata that stores the name of the person or company that created the document. It shows up in the file's properties and can be used to sort files in document libraries. Programs often fill it in automatically with your account name, so check or clear it before sharing a PDF publicly if you want to stay anonymous.

Also known as: Catalog, Root Dictionary, PDF Catalog

The document catalog is the starting point of every PDF's internal structure. The file's trailer points to it with a /Root entry, and the catalog points to the page list, bookmarks, form fields and metadata. A PDF viewer always reads the catalog first to find everything else. If the catalog is missing or damaged, the file usually will not open at all.

Also known as: Info dictionary, /Info dictionary, document info dictionary

The document information dictionary, or /Info dictionary, is the original place where a PDF stores its basic metadata. It holds simple fields such as Title, Author, Subject, Keywords, Creator, Producer, CreationDate and ModDate. Newer PDFs often store the same data in XMP as well. Many viewers still show these values in the document properties window, so check both places when clearing metadata.

Also known as: keywords, PDF keywords, document tags

Keywords are a PDF metadata field that holds a list of words or tags describing what the document is about, usually separated by commas or semicolons. Document management systems and internal search tools use them to find and group files. They help in large archives, but public search engines such as Google give them little weight, so the visible text matters more.

Also known as: file metadata, document properties

Document metadata is information stored inside a file that describes the file, rather than the text on its pages. In a PDF this includes the title, author, subject, keywords, dates and the software used, kept in the Info dictionary and XMP. Good metadata helps search and archiving. It can also reveal names or internal details, so review it before sharing a PDF.

Also known as: PDF title, title metadata

The Title is a PDF metadata field that holds the document's readable name, separate from its filename. Viewers can show it in the window or tab title, and search engines may use it when a PDF appears in results. A clear Title helps screen reader users and file libraries. Without it, people see a filename such as scan_0012.pdf instead.

Also known as: DOCX, XLSX, Office Open XML

DOCX and XLSX are the file formats used by Microsoft Word and Excel. Both are zipped bundles of XML files that store editable text, styles, tables and formulas. They are the usual targets when you convert a PDF back into an editable file. How good the result looks depends on the PDF, because fixed PDF pages must be rebuilt as flowing Word text.

Also known as: Tonal Value Increase, TVI

Dot gain is when printed ink dots spread on paper and come out larger than planned. Midtones get darker and colors shift. It is measured as a percentage increase at a given tint, also called tonal value increase. Print shops correct for it with tone curves or output profiles. Knowing the expected dot gain helps a printed PDF match the proof you saw on screen.

Also known as: DPI, PPI, pixels per inch, resolution

DPI, or dots per inch, is how many pixels are captured or printed across each inch of a page. Higher DPI means more detail and bigger files. For scanned PDFs, 300 DPI is the usual minimum for reliable OCR, while 150 DPI is fine for reading on screen. Doubling the DPI roughly quadruples the number of pixels and so the file size.

Also known as: Dublin Core Metadata Element Set, DCMI

Dublin Core is a standard set of 15 basic metadata fields for describing documents and other resources, including title, creator, subject, description, publisher and date. It is one of the default schemas inside a PDF's XMP metadata. Because libraries, archives and repositories all understand it, Dublin Core helps a PDF stay findable and correctly described when it moves between systems.

Also known as: eIDAS, eIDAS Regulation

eIDAS is a European Union regulation that sets common rules for electronic identification and trust services, such as electronic signatures, seals, timestamps and certificates, across all member states. It defines three signature levels, simple, advanced and qualified, and the legal weight of each. If you sign PDFs for use in the EU, eIDAS decides how strong the signature must be.

Also known as: EPUB, ePub, e-book

EPUB is an open e-book format built on HTML and CSS. Unlike a PDF, which has fixed pages, EPUB text reflows to fit any screen and font size. Converting a PDF to EPUB makes it easier to read on phones and e-readers. It works best with text-based, tagged PDFs, because the layout must be rebuilt as flowing text and complex pages can break.

Also known as: e-signature, esignature, electronic sign

An electronic signature is any electronic mark a person adds to a document to show they agree, such as a typed name, a drawn signature or a click on I agree. Laws like the EU's eIDAS and the US ESIGN Act recognize them. They let you sign a PDF without printing it. They are usually less secure than certificate-based digital signatures, which detect later changes.

Also known as: EPS, EPS file

Encapsulated PostScript (EPS) is an older file format that holds a single graphic, such as a logo or vector illustration, ready to be placed in another document. It is still common in print work. Most modern design tools and viewers handle PDF more reliably than EPS, so EPS files are often converted to PDF or embedded inside one before sharing or printing.

Also known as: Encrypt dictionary, /Encrypt

The encryption dictionary is the part of an encrypted PDF that describes how the file is protected. It records the encryption method, key length, permission settings and values used to check passwords. A PDF reader reads it first to work out the decryption key and to apply limits such as no printing. If it is damaged, the file cannot be opened.

Also known as: %%EOF, EOF marker, startxref

The %%EOF marker is the line that ends a PDF file, or ends each update section added to it. Just before it, the startxref line gives the position of the cross-reference table, which readers use to find everything else. If a download is cut short, the %%EOF marker is often missing, and viewers report the PDF as damaged or truncated.

Also known as: export, import, save as

Export and import mean moving content between file formats. Exporting writes a PDF's pages or data out to another format, such as Word, Excel or JPG images. Importing brings outside content, such as images or form data, into a PDF. Both let you reuse PDF text in an editor or move tables into a spreadsheet without retyping them.

Also known as: XMP, ISO 16684, XMP metadata

XMP (Extensible Metadata Platform) is a standard, ISO 16684, for storing metadata inside a file as XML. In a PDF it holds details such as title, author, copyright and which standards the file claims to meet. PDF/A requires an XMP packet stating its part and conformance level. Validators and archives read XMP to check and catalogue files automatically.

Also known as: Factur-X, ZUGFeRD, Hybrid Invoice

Factur-X and ZUGFeRD are the French and German names for the same e-invoice format. Each invoice is a PDF/A-3 file that people can read, with an XML file inside it holding the invoice data for software. Accounting systems can read the XML and book the invoice automatically. It helps businesses meet European e-invoicing rules while still sending a normal-looking PDF.

Also known as: FDF, Forms Data Format, .fdf

FDF (Forms Data Format) is a small file format that stores only the values typed into a PDF form, not the form itself. It lets you export answers from a form, import them into another copy or send them to a server. Because FDF files hold just the data, they are much smaller than the full PDF and useful for pre-filling forms.

Also known as: form validation, input validation

Field validation is a set of checks a PDF form runs on what you type. It can require a field, limit numbers to a range or check that a date or email has the right format. If the input fails, the form shows a warning and may refuse the value. Validation usually relies on JavaScript, so some browser and mobile viewers skip it.

Also known as: File Attachment, Attachment Annotation

A file attachment annotation is a file embedded in a PDF and pinned to a specific spot on a page, shown as a paperclip or pin icon. Clicking it lets the reader open or save the attached file. Unlike attachments listed for the whole document, it is linked to a passage, which is useful for attaching source data or evidence next to the text it supports.

Also known as: /ID, file ID, document ID

The file identifier is a pair of codes stored in a PDF's trailer under /ID. The first code is set when the file is created and stays the same. The second changes each time the file is saved with changes. Comparing them shows whether a file is an original or a revised copy. Encryption also uses the identifier when building the key.

Also known as: FlateDecode, zlib, Deflate, Flate

Flate is the most common compression method inside PDFs. It is the same DEFLATE method used by ZIP files, decoded with the FlateDecode filter. It is lossless, so the data comes back exactly as it was. Text, vector graphics and many images are stored with Flate, and compressing them again at a higher level is one way to make a PDF smaller.

Also known as: flatten PDF, flattening, flatten layers

Flattening a PDF means merging form fields, comments, stamps and layers into the page itself so they become fixed content. After flattening, nobody can edit or remove them as separate items. People flatten PDFs to lock in filled forms, signature images or watermarks before sending. Keep a copy of the original first, because flattening cannot be undone.

Also known as: flatten, transparency flattening, flatten PDF

Flattening means merging separate layers of a PDF into one fixed layer. In printing, transparency flattening turns see-through effects and blends into plain shapes or images that any printer can handle. For forms, flattening turns filled fields and comments into normal page content. Either way the file looks the same in every viewer, but those items can no longer be edited.

Also known as: FontDescriptor

A font descriptor is the part of a PDF that describes a font's measurements and style, such as letter height, italic angle and whether it is bold or serif. For embedded fonts it also points to the font file itself. If the font is not embedded, viewers use the descriptor to choose the closest matching font on the device.

Also known as: embedded fonts, embed font

Font embedding means storing the font itself inside the PDF, instead of relying on fonts installed on the reader's device. With embedded fonts, text looks exactly as designed on any computer, phone or printer. Without them, the viewer swaps in a similar font, which can change spacing and line breaks. Print shops and PDF/A both require embedded fonts.

Also known as: subset font, font subset

Font subsetting means embedding only the characters a PDF actually uses, not the whole font. A font can contain thousands of glyphs, so a subset is often far smaller and the text still looks exactly the same. You can spot subset fonts in a PDF's font list by a six-letter prefix, such as ABCDEF+Arial. It is a standard way to shrink PDFs.

Also known as: missing font, substituted font, font fallback

Font substitution happens when a PDF uses a font that is neither embedded in the file nor installed on your device, so the viewer swaps in a similar one. The replacement rarely has the same letter widths, so text can shift, wrap differently or overflow boxes. Embedding the fonts when you create the PDF is the reliable way to prevent it.

Also known as: PDF form field, fillable field

A form field is an interactive box in a PDF where you can type or choose an answer, such as a name, date, checkbox or signature. Each field has a name, a type and a value, and it shows on the page through a widget. Form fields let a PDF collect answers that can be saved, printed or exported as structured data.

Also known as: field calculation, calculated field, form calculation

A form field calculation fills in a field automatically from other fields, such as adding line items into a total or applying a tax rate. Calculations run in a set order, using simple functions such as sum or average, or JavaScript. They cut down on manual errors in invoices and order forms. Some browser and mobile viewers do not run them.

Also known as: flatten PDF form, flatten form fields

Form flattening turns a filled PDF form into a plain document. The typed answers are drawn onto the page permanently, and the interactive fields are removed. Nobody can then change the answers, and the form looks the same in every viewer, including ones that show fields badly. Keep an unflattened copy if you might need to edit the answers later.

Also known as: free text, text box annotation, typewriter

A FreeText annotation is a text box typed straight onto a PDF page, instead of hidden inside a pop-up note. It stores its own font, size, color and alignment. People use it to add visible labels or comments, or to fill in blanks on a form that has no fillable fields. The original page stays unchanged underneath, so the text can be moved or deleted.

Also known as: presentation mode, full screen

Full-screen mode shows a PDF across the whole screen with no toolbars, menus or window borders, like a slideshow. It is used for presentations and kiosks, where readers move through pages like slides, sometimes with page transitions. An author can set a PDF to open in full-screen automatically through its initial view settings, though many browser viewers ignore that setting.

Also known as: glyphs

A glyph is the actual shape drawn for a character in a particular font, such as the curves that form a lowercase 'a' in Arial. One character can have several glyphs, and some glyphs, such as ligatures, stand for several characters. A PDF draws text by picking glyphs from a font, so the link between glyphs and characters decides whether search and copy work.

Also known as: greyscale, 8-bit gray

A grayscale scan records each pixel as a shade of gray from black to white, with no color. It is much smaller than a color scan but keeps more detail than pure black and white, so faint pencil marks, stamps and photos stay visible. For text documents it is a good default, and it often gives OCR cleaner letter edges than harsh black-and-white conversion.

Also known as: screening, halftone screen, halftoning

A halftone is a pattern of tiny dots used in printing to create the look of smooth shades and colors. A press can only print each ink fully or not at all, so it varies the size or spacing of dots, and the eye blends them into tones. Screen frequency, measured in lines per inch, affects how sharp printed images in a PDF look.

Also known as: running head, page header, page footer

Headers and footers are text or images repeated in the top and bottom margins of each page, such as a title, date, file name, page number or confidentiality notice. They give readers context on every page without touching the main content. In PDFs they are often added after the file is created, for example to add page numbers or Bates numbers to a finished document.

Also known as: headings, H1 H2 H3, heading structure

Heading tags, H1 through H6, mark the titles and section headings in a tagged PDF and show how sections nest. Screen reader users use them to skim a document and jump between sections, the same way they move through headings on a web page. Headings in a correct order without skipped levels are a key accessibility requirement and also make the PDF easier to convert.

Also known as: metadata leakage, hidden data, residual metadata

Hidden metadata is information stored inside a PDF that does not show on the page but travels with the file. It can include author names, the software used, dates, edit history and custom properties. When you share a document, this data can reveal personal or internal details. Clearing hidden metadata is a standard step before publishing or sending a PDF outside your organization.

Also known as: highlight, text highlight

A highlight annotation is a see-through block of color placed over text to mark it, usually yellow. In the PDF it is stored as a set of corner points that trace the covered words. The text underneath stays readable, selectable and searchable. Highlights are added on top of the page, so you can delete them later or remove all annotations at once.

Also known as: hocr, OCR HTML

hOCR is an open format that stores OCR results as HTML. Each recognized word, line and block keeps its exact position on the page, and often a confidence score. OCR engines such as Tesseract can output it, and other tools use it to correct the text or to add an invisible, searchable text layer to a scanned PDF in exactly the right place.

Also known as: HTML-to-PDF, web page to PDF

HTML to PDF conversion takes a web page, with its styles and images, lays it out like a browser would and saves the result as a PDF with fixed pages. It is often used to create invoices, reports and receipts from web templates. The tricky parts are controlling where pages break, using print styles and keeping fonts and dynamic content intact.

Also known as: link annotation, link, URL link

A hyperlink in a PDF is a clickable area on a page that opens a web address, jumps to another page in the document or runs an action. It is stored as a link annotation. Hyperlinks let a table of contents, cross-references and citations take readers straight to their target. Flattening or printing a PDF to an image removes them.

Also known as: ICC profile, color profile

An ICC color profile is a small data file, defined by the International Color Consortium, that describes how a specific device or color space shows color. Embedding profiles in a PDF lets screens, printers and presses translate its colors consistently. Print standards such as PDF/X and archive standards such as PDF/A rely on them to keep colors predictable.

Also known as: downsampling, resampling, resolution reduction

Downsampling lowers the resolution of images inside a PDF, for example from 600 DPI to 150 DPI, by merging pixels before saving them again. Images usually make up most of a PDF's size, so matching their resolution to how the file will be used is one of the best ways to compress it. Go too low and photos and scanned text look soft or blurry.

Also known as: scanned pdf, image pdf

An image-only PDF is a PDF where each page is just a picture, with no real text inside. Scanners and photo-to-PDF apps usually produce these. You cannot select, copy or search the words until OCR adds a text layer. Image-only PDFs are also larger than text-based ones and cannot be read aloud by screen readers.

Also known as: page imposition

Imposition is arranging several pages on one large print sheet so that, after printing, folding and cutting, they come out in the right order and the right way up. Imposition software turns a normal page-by-page PDF into printer spreads with room for binding and trimming. It saves paper and is needed for booklets, magazines and other multi-page print jobs.

Also known as: incremental save, incremental saving, incremental update

An incremental update is a way of saving changes to a PDF by adding them to the end of the file, while leaving the original content untouched. This makes saving fast and keeps earlier digital signatures valid. The catch is that deleted or replaced content can still be present in the file, so a full re-save is needed to remove it for good.

Also known as: indirect reference, object reference

An indirect object is a numbered object inside a PDF file, written as 'N G obj ... endobj', where N is the object number and G its generation number. Other parts of the file refer to it as 'N G R'. This lets one object, such as a font or image, be stored once and reused on many pages. The cross-reference table lists where each one sits.

Also known as: open action, document open settings, initial display

The initial view is a set of options that control how a PDF looks when it first opens, such as the starting page, zoom level, page layout and which side panels are shown. Authors use it to open a document on its cover, fit it to the window or start in full-screen. Desktop readers usually respect it, but many browser and phone viewers ignore it.

Also known as: freehand annotation, pen annotation, drawing annotation

An ink annotation is a freehand drawing on a PDF page, made with a mouse, pen or finger. It is stored as one or more lines of connected points with a color and width. Ink is used for handwritten notes, circles and arrows, and for drawn signatures. Like other annotations it sits on top of the page and can be deleted or flattened.

Also known as: ICR, Handwriting Recognition

ICR, or Intelligent Character Recognition, is a type of OCR that reads handwriting. It uses machine learning trained on many writing styles. It works best on hand-printed forms where each letter sits in its own box. Accuracy is lower than OCR on printed text, so ICR results usually need a human check before the data from a scanned PDF is used.

Also known as: ISO 32000-1, PDF specification

ISO 32000 is the international standard that defines the PDF format. Adobe handed the specification to the International Organization for Standardization, which published it as ISO 32000-1 in 2008. ISO 32000-2, also known as PDF 2.0, followed in 2017. Because the format is an open standard, PDFs from any compliant software open in any compliant reader.

Also known as: JBIG2, JBIG2Decode

JBIG2 is a compression method for black-and-white scanned pages, decoded in PDF by the JBIG2Decode filter. It stores repeated shapes, such as each letter, once and reuses them, so files are much smaller than with CCITT Group 4. It has lossless and lossy modes. The lossy mode can swap similar characters, such as 6 and 8, so use it with care on numbers.

Also known as: JPEG 2000, JPXDecode, JPX, JP2

JPEG 2000 is an image format that PDFs can use through the JPXDecode filter. Unlike standard JPEG, it uses wavelets, which avoid blocky artifacts and give better quality at small sizes. It supports both lossy and lossless compression, high bit depths and transparency. Fewer tools support it than ordinary JPEG, so it mostly appears in archival or high-quality scanned PDFs.

Also known as: kern, kerning pairs

Kerning is adjusting the space between certain pairs of letters, such as A and V, so the spacing looks even. In a PDF, kerning is applied by placing each glyph with small offsets when the text is drawn, not by changing the font. Good kerning helps readability. Very tight offsets can also make copied or extracted text come out with missing or extra spaces.

Also known as: LZW, LZWDecode, Lempel-Ziv-Welch

LZW is a lossless compression method used in older PDFs through the LZWDecode filter, and also in GIF and TIFF images. It builds a table of repeated byte patterns and replaces repeats with short codes. Flate compresses better and has replaced it. Converting old LZW streams to Flate is a simple way to shrink a legacy PDF without changing its content.

Also known as: ligatures, typographic ligature

A ligature is a single glyph that joins two or more letters into one shape, such as 'fi' or 'fl', for neater spacing. In a PDF, a ligature may be stored as one glyph for several letters. If the file lacks a ToUnicode mapping, copying or searching that word can fail, so 'find' might paste out as 'nd' or a strange symbol.

Also known as: Line Markup

A line annotation is a straight line drawn between two points on a PDF page. It can have arrowheads, a caption or leader lines. Because it sits on top of the page, you can move, restyle or delete it without changing the content underneath. Reviewers use it to point at details, mark distances on drawings or connect a comment to an area.

Also known as: fast web view, linearized pdf, web optimized pdf

A linearized PDF, also called Fast Web View, is a PDF arranged so the first page can display before the whole file has downloaded. The rest of the pages load as you scroll. It helps with large PDFs on websites or slow mobile connections. Editing and saving the file in many tools can undo linearization, so apply it as the last step.

Also known as: hyperlink, PDF link

A link annotation is a clickable area on a PDF page that jumps to another place in the document or opens a web address. It is stored as a rectangle plus a destination or action. Link annotations make tables of contents, cross-references and URLs work as real links. Some tools that remove annotations also remove links, so check the option before you run them.

Also known as: Listbox, List Field

A list box is a PDF form field that shows several options in a scrolling list. The user picks one option or, if the form allows it, several. Unlike a dropdown (combo box), the choices are always visible, and typing a custom answer is not allowed. List boxes suit forms that need answers from a fixed set of values, such as choosing departments.

Also known as: structure tree, structure hierarchy, logical structure

The logical structure tree is the outline of a tagged PDF, listing its headings, paragraphs, lists, tables and figures in reading order. It is stored under the StructTreeRoot. Screen readers follow this tree to read the document in the right order and announce what each part is. Conversion tools also use it, so tagged PDFs convert to Word or EPUB more cleanly.

Also known as: LTV, long term validation

Long-Term Validation (LTV) means storing everything needed to check a digital signature inside the signed PDF itself, including certificate chains, revocation checks and timestamps. Certificates expire and online check services can disappear. With LTV the signature can still be verified years later, with no outside servers. Contracts and records that must be kept for a long time usually need it.

Also known as: lossy compression, lossless compression

Lossless compression shrinks data so it can be restored exactly, bit for bit, as Flate and CCITT do. Lossy compression, such as JPEG, throws away detail you are unlikely to notice to reach much smaller sizes. When compressing a PDF, lossless suits text and diagrams, while lossy suits photos. Lossy changes cannot be undone, so keep the original if quality matters.

Also known as: Marked Content Sequence, BDC, EMC

Marked content is a way of labeling pieces of a page's drawing instructions inside a PDF, using the BMC, BDC and EMC operators. These labels connect text and images on the page to entries in the structure tree, so a screen reader knows which words form a heading, paragraph or table cell. Marked content is the low-level base of every tagged, accessible PDF.

Also known as: markup, text markup

A markup annotation is any annotation used to review a document, such as highlights, underlines, strikeouts, sticky notes and freehand drawings. Each one can store an author, a date and replies, so comments can form threads. Several people can mark up the same PDF without changing its content, and all markup can be removed before the final version is shared.

Also known as: Matterhorn, PDF/UA checklist

The Matterhorn Protocol is a checklist from the PDF Association that turns the PDF/UA accessibility standard into 31 checkpoints and 136 failure conditions. Testers and developers use it to check whether a tagged PDF is accessible. Some conditions can be checked by software, but many need a person to judge, such as whether alt text actually describes the image.

Also known as: media box

The MediaBox is the required page box that sets the full size of a PDF page, in points (1/72 inch). An A4 page has a MediaBox of 595 by 842 points, and US Letter is 612 by 792. Other page boxes, such as the CropBox, should fit inside it. Printers and viewers use it to know the paper size.

Also known as: Combine PDF, Join PDF, Merge

Merging a PDF means combining two or more PDF files, or chosen pages from them, into one document in the order you pick. It is useful for gathering chapters, scanned receipts or signed contracts into a single file that is easier to send, store and print. A good merge keeps the original page quality, and where possible it also keeps bookmarks and links.

Also known as: XMP packet, metadata stream object

A metadata stream is the part of a PDF that holds its XMP metadata as an XML block, linked from the document catalog. It is the modern home for PDF metadata, next to or instead of the older Info dictionary. Because it is plain XML, tools can find and update it easily. PDF/A files must have one, so validators check it.

Also known as: MRC, layered compression

Mixed Raster Content (MRC) is a way of compressing scanned color pages by splitting each page into layers. Typically a sharp black-and-white text layer sits over a lower-resolution color background, and each layer gets the best compression for it. Text stays crisp while backgrounds and photos take little space, so a color scan can become many times smaller.

Also known as: n-up, multiple pages per sheet, 2-up

N-up layout means putting several PDF pages on one sheet, such as 2 or 4 pages per side, by shrinking and arranging them in a grid. It saves paper when printing and is handy for handouts or proof copies. Each page is printed smaller, so 4-up on A4 suits slides more than dense text, which may become hard to read.

Also known as: pdf name, name object, name

A name object is an identifier in a PDF file that starts with a slash, such as /Type or /MediaBox. Names are used as keys and labels inside the file's structure, not as text shown on the page. Special characters in a name are written as #xx hex codes. You only see names when you open a PDF's raw code or work with it programmatically.

Also known as: named dest, destination, named target

A named destination is a label for a specific spot in a PDF, set by page, position and zoom. Links and bookmarks can point to the name instead of a fixed page number, so they keep working when pages are added or removed. You can also link to a named destination from a web page by adding it to the PDF's URL after a # sign.

Also known as: nonrepudiation

Non-repudiation means a signer cannot believably deny that they signed a document. Digital signatures provide it because they are made with a private key that only the signer controls, and anyone can check them with the signer's certificate. For signed PDFs, this is what gives contracts and approvals legal weight. Simple electronic signatures, such as a typed name, offer much weaker proof.

Also known as: ObjStm, compressed object stream, object stream

An object stream (ObjStm) is a feature added in PDF 1.5 that bundles many small objects in a PDF into one compressed block. This can cut file size a lot, especially in files with many small objects. Objects inside a stream can only be found through a cross-reference stream, so very old PDF readers may fail to open these files.

Also known as: object stream, ObjStm, compressed object stream

Object stream compression packs many small PDF objects into one Flate-compressed stream instead of storing each one as plain text. It was added in PDF 1.5 and works together with cross-reference streams. It reduces the file's structural overhead without touching images or page content, so it shrinks files with many objects, such as tagged documents or large forms, with no visible change.

Also known as: OCSP

OCSP (Online Certificate Status Protocol) is a way to ask a Certificate Authority, in real time, whether one specific certificate has been revoked. It is lighter than downloading a full revocation list. When a PDF signature is checked, an OCSP response confirms the signer's certificate was valid. Storing that response in the PDF supports long-term validation after the certificate expires.

Also known as: OpenType, OTF

OpenType is a modern font format that works on Windows, macOS and Linux. One OpenType file can hold either TrueType or PostScript-style outlines, and it supports large Unicode character sets and features such as ligatures. OpenType fonts use the .otf or .ttf extension. When a PDF is created, they are usually embedded as a subset so the text looks right everywhere.

Also known as: OCR, text recognition

Optical Character Recognition (OCR) is software that reads the text in an image and turns it into real, editable characters. Applied to a scanned PDF, OCR adds an invisible text layer behind the page image. You can then search, select and copy the words, and screen readers can read them. Accuracy depends on scan quality, so clean 300 DPI scans give the best results.

Also known as: OMR, Mark Recognition

Optical Mark Recognition (OMR) detects whether a mark is present at fixed spots on a scanned form, such as a filled bubble or a ticked box. It does not read letters or numbers. It is used for surveys, ballots and multiple-choice tests. Because it only checks for marks, OMR is fast and very accurate, and answers from scanned PDF forms can be counted automatically.

Also known as: OCG, PDF layers, optional content, layer

An optional content group, usually called a layer, is a part of a PDF that can be shown or hidden on its own from the viewer's layers panel. Layers are common in CAD drawings, maps and multilingual files, where you might switch dimensions or a language on and off. Flattening a PDF merges all visible layers into the page permanently.

Also known as: ICC output intent, output intents

An output intent is information inside a PDF that says which printing or display conditions the colors were prepared for, usually by including an ICC color profile. PDF/X always requires one, and PDF/A requires one when the file uses device color. It tells software how to convert the file's colors for a specific press or screen. Without it, colors in printed or archived files can shift unpredictably.

Also known as: overprinting

Overprint is a print setting where one ink is printed directly on top of another, instead of leaving a gap, or knockout, under it. It is often used for black text on colored backgrounds, so small misalignments between printing plates do not leave white edges. Wrong overprint settings can change colors or make white objects disappear in print, so check them with an overprint preview.

Also known as: permissions password, master password

An owner password, also called a permissions password, controls what people can do with a PDF, such as printing, copying text or editing. It does not stop anyone from opening the file. Readers that follow the rules enforce these limits, but some tools ignore them. Entering the owner password, or removing it with an unlock tool, gives full access again.

Also known as: page numbering, page labels

A page label is the page number a PDF viewer shows for a page, which can differ from its actual position in the file. Labels can use Roman numerals, letters or prefixes, and can restart partway through. A book with front pages numbered i to x can then label page 11 of the file as page 1, matching the printed numbers.

Also known as: continuous scrolling, two-page view, facing pages

Page layout is the viewer setting that controls how pages are arranged on screen: single page, continuous scrolling, two pages side by side, or two pages with a separate cover. Continuous scrolling suits reports, while facing pages suit books and magazines. A PDF can store a preferred layout to use when it opens, though browser and phone viewers often ignore it.

Also known as: PageMode, display mode

Page mode is a PDF setting that tells the viewer which side panel to show when the file opens, such as bookmarks, page thumbnails, attachments, layers or none. Authors use it to open a long report with its bookmarks already visible, or to hide every panel for clean reading. Desktop readers respect it more often than browser or mobile viewers.

Also known as: pagination, page numbers

Page numbering, or pagination, means numbering the pages of a document in order, so readers can find and cite pages and see that nothing is missing. Numbers can be 1, 2, 3 or i, ii, iii and usually sit in a header or footer. After merging or reordering PDFs, adding page numbers again keeps the final file easy to reference and print.

Also known as: portrait, landscape, page rotation

Page orientation is whether a page is taller than it is wide, called portrait, or wider than it is tall, called landscape. Scanned pages often come out sideways or upside down, and wide tables read better in landscape. Rotating a PDF page in 90-degree steps fixes the orientation, so the document shows the right way up on screen and prints correctly.

Also known as: rotate, page orientation, Rotate attribute

Page rotation is a setting on each PDF page that tells viewers to turn it clockwise by 90, 180 or 270 degrees. It changes how the page is shown and printed without redrawing its content, so quality is not affected. It is the quick fix when a PDF is upside down or sideways, or when you need to switch a page between portrait and landscape.

Also known as: page dimensions, A4, Letter, paper size

Page size is the width and height of a PDF page, set by its MediaBox. The most common sizes are A4, 210 by 297 mm, used in Europe and most of the world, and US Letter, 8.5 by 11 inches. Mixing sizes in one file can cause odd scaling or margins when printing, so resizing all pages to A4 or Letter keeps output consistent.

Also known as: thumbnails, thumbnail panel, page preview

A page thumbnail is a small preview image of a PDF page, usually shown in a grid or sidebar. Tapping a thumbnail jumps to that page. Thumbnails let you see the layout of a long document at a glance. In tools that organize PDFs, you drag thumbnails to reorder pages, or select them to delete or rotate pages without opening each one.

Also known as: slide transition, transition effect

A page transition is a visual effect, such as a dissolve, wipe or fly-in, that a PDF viewer plays when moving to the next page in full-screen mode. It is used to make PDFs behave like slideshows. Each page can have its own transition and an automatic advance time, so the document can play by itself. Most browser viewers ignore transitions.

Also known as: page node tree, pages dictionary

The page tree is the internal structure that lists every page of a PDF, starting from the document catalog's Pages entry. Pages hang off it like leaves, grouped by inner nodes. Some settings, such as page size and rotation, can pass down the tree. Tools that reorder, delete or insert pages must update this tree correctly or pages go missing.

Also known as: Pantone, PMS

The Pantone Matching System (PMS) is a numbered library of standard ink colors, such as Pantone 186 C, used to print an exact color the same way anywhere. In a PDF, Pantone colors appear as named spot colors, or are converted to the closest CMYK mix. Using Pantone codes keeps brand colors consistent across printers and papers, though CMYK conversion can shift them.

Also known as: password protect, user password, owner password

Password protection locks a PDF behind a password. A user password, or open password, is needed to open the file at all. An owner password limits actions such as printing or editing. Both work on top of encryption. Protecting a PDF keeps sensitive files private when you email or share them, and if you know the password you can remove it later.

Also known as: ISO 32000-2, PDF 2

PDF 2.0 is the latest version of the PDF format, published as ISO 32000-2 in 2017. It is the first version written entirely by ISO committees rather than by Adobe. It improves encryption, digital signatures, tagging and metadata, and removes unclear older features. Newer standards such as PDF/UA-2 build on it, though most PDFs in use today are still version 1.x.

Also known as: PAdES

PAdES (PDF Advanced Electronic Signatures) is the ETSI standard for advanced and qualified electronic signatures inside PDF files. It builds on the normal PDF signature format and adds support for timestamps and long-term validation, meeting the EU's eIDAS rules. Certificate and revocation data are stored inside the file, so a PAdES-signed PDF can still be verified years after signing.

Also known as: Creator, PDF creator field

The Creator is a PDF metadata field that names the program used to make the original document, such as Microsoft Word or InDesign, before it was turned into a PDF. It differs from the Producer, which names the software that wrote the PDF itself. Together they show how a file was made. They can also reveal internal tools, so check them before sharing.

Also known as: encrypt PDF, document encryption

PDF encryption scrambles a document's content so it can only be read with the right password or key. The method and key length are stored in the file's encryption dictionary, and may be RC4, AES-128 or AES-256. RC4 is outdated, so use AES-256 for anything sensitive. Encryption keeps a PDF private when it is emailed, shared or stored.

Also known as: %PDF header, file header, %PDF-1.7

The PDF header is the first line of every PDF file, such as %PDF-1.7 or %PDF-2.0, stating which PDF version the file uses. It is often followed by a line of special bytes that tells other software to treat the file as binary. If the header is missing or damaged, many tools refuse to recognize the file as a PDF.

Also known as: Compress PDF, Optimize PDF, Reduce PDF Size

PDF optimization, often called compressing a PDF, makes the file smaller without changing what it says. Common steps include lowering image resolution, recompressing images, keeping only the font characters that are used and removing duplicate or unused data. Smaller files are easier to email or upload under size limits. Strong compression can make images look softer, so check the result.

Also known as: unlock PDF, decrypt PDF, remove PDF password

PDF password removal, or decryption, takes a protected PDF and saves an unlocked copy that opens with no password and no limits on printing or copying. You need to know the password, since removal is not cracking. It helps when you have the right to use a file but need to print, edit or merge it often without typing the password each time.

Also known as: PDF Package, Portfolio, Collection

A PDF portfolio is a single PDF that holds several separate files, such as PDFs, spreadsheets and images, each kept in its original format. Readers see a cover page and can open each file on its own. Unlike merging, which joins pages into one document, a portfolio keeps the files apart. Many browser and phone viewers cannot display portfolios properly.

Also known as: Producer, PDF producer field

The Producer is a PDF metadata field that names the software that actually wrote the PDF file, such as a PDF printer driver or a conversion library. It differs from the Creator, which names the program the content was first made in. Comparing the two helps trace how a file was made and troubleshoot display or conversion problems.

Also known as: Acrobat compatibility, PDF 1.4, PDF 1.7

PDF version compatibility is about which version of the PDF format a file uses, from 1.4 up to 2.0, often labeled by the matching Acrobat release. Newer versions add features such as AES-256 encryption, layers and better tagging. Very old readers may not open them correctly. Choosing a version means balancing those features against the software your recipients use.

Also known as: PDF/A, ISO 19005, archival PDF

PDF/A is a version of PDF made for long-term archiving, defined in the ISO 19005 standards. It only allows features that will display the same way decades from now. Fonts and color information must be embedded, and encryption, JavaScript and links to outside content are not allowed. Courts, governments and libraries often require PDF/A for documents that must be kept.

Also known as: PDF/A-1a, PDF/A-1b, PDF/A-2b, level A B U

PDF/A conformance levels show how strictly a file meets the archiving standard. Level B (basic) guarantees the pages look the same in the future. Level U also requires Unicode text, so it can be searched and copied. Level A goes further and requires a tagged structure for accessibility. If a rule asks for PDF/A, check which level it needs.

Also known as: PDF/A-3, ISO 19005-3, Archival PDF

PDF/A-3 is the part of the PDF/A archiving standard, ISO 19005-3, that allows any kind of file to be embedded inside the PDF. Earlier parts did not. This means an archived PDF can carry its source spreadsheet or an XML data file. It is the basis of e-invoices such as Factur-X and ZUGFeRD, where the readable invoice includes its machine data.

Also known as: PDF/E, ISO 24517

PDF/E is the ISO 24517 standard version of PDF for engineering, manufacturing and construction documents. It supports large technical drawings and interactive 3D models. Teams can share and review designs without the original CAD software, and keep a stable record of the design. It is far less common than PDF/A or PDF/X, and mostly used in specialist engineering workflows.

Also known as: PDF/UA, ISO 14289, accessible PDF

PDF/UA is the ISO 14289 standard for accessible PDFs, so screen readers and other assistive tools can use them reliably. It requires a tagged PDF with a correct reading order, alt text on images, real headings and properly marked tables. Laws such as the European Accessibility Act, the ADA and Section 508 often lead organizations to target PDF/UA.

Also known as: PDF/VT, ISO 16612-2

PDF/VT is the ISO 16612-2 standard for printing large runs of personalized documents, such as bank statements, invoices and direct mail. It builds on PDF/X and marks which graphics repeat, such as a logo, so the press can process them once and reuse them. This lets printers produce millions of unique pages quickly and reliably.

Also known as: PDF/X, ISO 15930, PDF/X-4

PDF/X is a family of ISO 15930 standards for sending PDFs to commercial printers. It requires embedded fonts and a stated output intent for color, and it bans features a press cannot handle reliably. PDF/X-4 is the most common version today. Sending a PDF/X file reduces surprises such as missing fonts or color shifts, and the costly reprints that follow.

Also known as: PDF permissions, usage permissions, P value

Permission flags are settings stored in an encrypted PDF that say which actions are allowed, such as printing, high-quality printing, copying text, editing, adding comments or filling forms. They work together with the owner password. A file can open freely and still block copying or printing. Compliant readers follow the flags, but they are not strong protection on their own.

Also known as: PNG vs JPEG, JPG vs PNG

PNG and JPG are the two most common image formats. PNG is lossless and supports transparency, so it suits text, screenshots and diagrams. JPG is lossy and makes photos much smaller with a small loss in quality. When you turn PDF pages into images, choose PNG for sharp text and JPG for photos or smaller files.

Also known as: points, pt, PDF unit

A point is the basic unit of measurement in a PDF, equal to 1/72 of an inch, or about 0.353 mm. Page sizes, margins and positions are all measured in points. An A4 page is 595 by 842 points, and US Letter is 612 by 792 points. Converting to millimeters or inches helps when you set exact sizes or positions.

Also known as: popup, comment popup

A popup annotation is the small window that opens to show the text of a comment, such as a sticky note or a highlight with a note. It always belongs to another annotation and has no meaning on its own. Popups keep comment text hidden until you click it, so the page stays readable while reviewers can still leave long notes.

Also known as: PDF/A, PDF for archiving

PDF/A is an ISO standard version of PDF for long-term archiving. Fonts must be embedded, colors must be device-independent or tied to an embedded ICC profile, and encryption and outside links are not allowed, so the file looks the same decades from now. Legal, government and records teams often require it, and many tools can save or convert a document to PDF/A.

Also known as: portrait, landscape, page orientation

Portrait means a page is taller than it is wide, and landscape means it is wider than it is tall. In a PDF, the orientation you see comes from the page size plus any rotation setting. Text documents are usually portrait, while wide tables and slides suit landscape. To convert a page from landscape to portrait or back, rotate it by 90 degrees.

Also known as: PS, PostScript language

PostScript is a page description language created by Adobe that tells printers exactly how to draw text, shapes and images on a page. PDF grew out of PostScript as a simpler, self-contained format for sharing files. Older print workflows still produce .ps files, which are usually converted, or distilled, to PDF so they can be viewed and printed reliably.

Also known as: Type 1, Type 1 font, PostScript font

PostScript Type 1 is an older Adobe font format that was the standard in professional printing for many years. Adobe has ended support for Type 1 fonts in its current apps, so they are being replaced by OpenType. Many older PDFs still contain embedded Type 1 fonts. These still display correctly, because the font is stored inside the file.

Also known as: preflight check, preflighting

Preflight is an automatic check of a PDF against a set of rules before it is printed or archived. It flags problems such as missing fonts, low-resolution images, wrong color settings or breaks from standards like PDF/X and PDF/A. The name comes from a pilot's pre-flight checklist. Catching problems early avoids failed print jobs and rejected submissions.

Also known as: print-to-PDF, save as PDF

Print to PDF is a printer option that saves whatever you print as a PDF file instead of sending it to paper. It is built into Windows, macOS, iOS, Android and most browsers. The result keeps the layout and page breaks you would get on paper. It works from almost any app, including ones with no Save as PDF option.

Also known as: Crop Marks, Registration Marks, Trim Marks, Printers Marks

Printer's marks are guide symbols placed outside the page area of a print-ready PDF. Crop marks, also called trim marks, show where to cut. Registration marks help line up each ink. Color bars and page details help check quality. They sit in the bleed or slug area and are cut away, so they never appear on the finished piece.

Also known as: PKI

Public Key Infrastructure (PKI) is the system of certificate authorities, certificates and rules that links digital keys to real, verified identities. Digital signatures depend on it. When you open a signed PDF, PKI supplies the chain of certificates and the revocation data your viewer uses to check that the signer is who they claim to be and is trusted.

Also known as: certificate security, PKI encryption, public key security handler

Public-key security, or certificate encryption, protects a PDF for specific people using their digital certificates instead of a shared password. Only someone holding the matching private key can open the file, and each recipient can get different permissions. It suits companies where sharing passwords is risky, but recipients need a certificate installed, which most home users do not have.

Also known as: command button, action button

A push button is a PDF form field that does something when clicked instead of storing an answer. Common uses are submitting a form, clearing all fields, printing or running a script. It can show a label or an icon. Many browser and mobile PDF viewers do not run button scripts, so submit buttons may do nothing outside Adobe Acrobat or Reader.

Also known as: QES, Qualified Signature

A Qualified Electronic Signature (QES) is the highest level of electronic signature under the EU's eIDAS regulation. It is made with a certified signing device and a qualified certificate from an approved trust service provider. A QES has the same legal effect as a handwritten signature across the EU, so it is used for contracts and official filings that require one.

Also known as: radio button group, option button

A radio button is a PDF form field where you pick exactly one option from a group, such as a payment method. All buttons in the group share one field name, so selecting one clears the others. Each option has its own value that is saved when the form is exported. This keeps answers clean, with no double choices.

Also known as: bitmap, raster, pixel image

A raster image, or bitmap, is a picture made of a fixed grid of colored pixels, unlike vector graphics, which are made of shapes. Scanners and cameras produce raster images, so every scanned PDF page is a bitmap. This is why zooming into a scan looks blocky, why resolution affects OCR and why scanned PDFs are larger than text-based ones.

Also known as: RIP, raster image processing

A Raster Image Processor (RIP) is the software or hardware that turns a PDF or PostScript file into the dot pattern a printer actually puts on paper or plate. It also separates colors, creates halftones and applies trapping. Different RIPs can treat fonts, transparency and overprint differently, so printers often test PDFs through their own RIP before running a job.

Also known as: Usage Rights, Reader-Enabled PDF, Reader Extensions

Reader Extensions, also called usage rights, are special permissions Adobe can add to a PDF. They let the free Adobe Reader do things it normally could not in older versions, such as saving filled forms, adding comments or signing. The rights are protected by an Adobe signature, so editing the file in other software can break them and trigger warnings in Reader.

Also known as: logical reading order, tag order

Reading order is the sequence in which a screen reader, or a tool that reflows text, reads a PDF's content. In a tagged PDF it follows the order of tags, not where things appear on the page. Multi-column layouts, sidebars and captions often end up in the wrong order. Checking the reading order makes sure the content makes sense when read aloud.

Also known as: redact PDF, black out text

Redaction is permanently removing sensitive text, images or data from a PDF, usually leaving a black box where it was. True redaction deletes the content from the file. Drawing a black shape over text does not, because the words can still be copied or extracted. Use a proper PDF redaction tool, and check that the metadata is also cleared.

Also known as: redaction, redact mark

A redaction annotation is a marker placed over text or images in a PDF to show what will be removed. Until you apply it, nothing is deleted, so you can review and adjust the marked areas. Applying the redaction deletes the content underneath for good and draws a solid box in its place. Save the result as a new file and keep the original elsewhere.

Also known as: Reflow View, Liquid Mode

Reflow rearranges a PDF's text into one continuous column that wraps to the width of the screen, instead of showing the fixed page. It makes PDFs much easier to read on phones and helps people who zoom in a lot. Reflow depends on the file's tags and reading order, so untagged PDFs often reflow badly or not at all.

Also known as: Rendering Intents, ICC Intent

A rendering intent is the rule used to convert colors that a printer or screen cannot reproduce exactly. There are four ICC intents: Perceptual, Relative Colorimetric, Absolute Colorimetric and Saturation. Each makes a different trade-off between keeping colors accurate and keeping them looking natural together. The choice matters most when you convert a PDF from RGB to CMYK for print.

Also known as: Resources, Resource Dict

A resource dictionary is the list of fonts, images, color settings and other items a PDF page needs to draw itself. Each page, or reusable graphic, points to its own resources. When PDFs are merged or edited, duplicate or unused resources can pile up and make the file larger, so cleaning them up is a common step when optimizing a PDF.

Also known as: RGB, RGB color

RGB is the color system used by screens, cameras and phones. It creates colors by mixing red, green and blue light. RGB can show bright, vivid colors, which is why it is the default for PDFs read on screen. Files sent to a commercial printer usually need converting to CMYK, because ink cannot reproduce every RGB color.

Also known as: built black

Rich black is a deep black made by printing black ink on top of some cyan, magenta and yellow, instead of using black ink alone. It looks darker and richer on large solid areas. It should not be used for small text, which can blur if the plates shift. Too much total ink can also cause smudging or slow drying.

Also known as: RC4, ARC4

RC4 is an old encryption method used by early PDF password protection, with 40-bit or 128-bit keys. It has known weaknesses, and PDF 2.0 deprecated it in favor of AES. PDFs locked with 40-bit RC4 can be cracked in minutes. If you have sensitive files protected this way, protect them again with AES-256.

Also known as: role mapping, RoleMap

A role map is a table in a tagged PDF that links custom tag names to the standard tag types, such as headings, paragraphs or lists. Some programs create their own tags, like ChapterTitle. The role map tells screen readers that ChapterTitle means a heading. Without it, assistive technology may not understand the custom tags and the document becomes harder to navigate.

Also known as: RunLength, RunLengthDecode, RLE

Run-length encoding (RLE) is a simple lossless compression method that replaces a run of the same byte with a count and the value, so 100 white pixels become one short instruction. In PDF it uses the RunLengthDecode filter. It only helps with simple graphics that have long repeated runs. Flate almost always compresses better, so modern PDFs rarely use RLE.

Also known as: metadata removal, PDF sanitizing, scrubbing

Sanitizing a PDF means removing hidden data that could leak information or carry harmful code. This includes metadata, JavaScript, embedded files, comments, form data and leftovers from earlier edits. A page can look clean and still reveal author names, edit history or old text. Sanitize files before you publish them or send them outside your organization.

Also known as: SVG

SVG (Scalable Vector Graphics) is an image format that describes shapes, lines and text in XML, so it stays sharp at any size. PDF also uses vector drawing, so converting between SVG and PDF can keep crisp lines and real text. SVG is handy for logos or for pulling diagrams out of a PDF to use on a website.

Also known as: scan to pdf, paper to pdf

Scan-to-PDF means capturing paper pages with a scanner or phone camera and saving them as a PDF. The result is usually an image-only PDF that keeps the look of the paper, but its text cannot be selected or searched. Running OCR afterwards adds a hidden text layer. For best results, scan at 300 DPI and keep pages flat and straight.

Also known as: screenreader, JAWS, NVDA, assistive technology

A screen reader is software that reads on-screen text aloud or sends it to a braille display, mainly for blind and low-vision users. Popular ones include JAWS, NVDA and VoiceOver. To read a PDF well, a screen reader needs tags, a correct reading order and alt text for images. Untagged or scanned PDFs are often unreadable to it.

Also known as: searchable image, sandwich pdf

A searchable PDF is a scanned document that keeps the original page image and adds an invisible text layer created by OCR. It looks exactly like the paper original, but you can search for words, select them and copy them. Screen readers can also read the text. Turning scans into searchable PDFs makes archives much easier to use.

Also known as: 508, 508 compliance

Section 508 is a US federal law that requires government agencies to make their electronic content accessible to people with disabilities. Its current standards follow WCAG 2.0 Level AA. For PDFs, this means proper tags, a logical reading order, alt text for images and accessible forms. US government bodies and their contractors usually must meet it.

Also known as: self signed certificate

A self-signed certificate is a digital certificate that the owner signed with their own key, instead of having it issued by a trusted Certificate Authority. It can still create valid signatures, but nobody independent has confirmed who the owner is. PDF viewers therefore show such signatures as untrusted unless the recipient manually trusts the certificate. It is common for internal use and testing.

Also known as: signature form field

A signature field is a spot in a PDF form reserved for a signature. It holds the visible signature, such as a name or image, and for digital signatures it stores the signature data and which part of the file is covered. Authors can place empty signature fields so signers know exactly where to sign.

Also known as: Split, Extract Pages, Divide PDF

Splitting a PDF means dividing one file into several smaller ones. You can extract a range of pages, save every page as its own file or split at fixed intervals. It is useful when you only need part of a large document, want to send chapters separately or must get under an upload size limit. The pages themselves stay unchanged.

Also known as: spot ink, spot colour

A spot color is a premixed ink printed from its own plate, rather than mixed from the four CMYK process inks. It gives an exact, consistent color across a print run. In a PDF, spot colors are stored as named Separation colors. They are used for brand colors, metallic and fluorescent inks, and for cheaper one-color or two-color print jobs.

Also known as: stamp, rubber stamp

A stamp annotation is a graphic placed on a PDF page like a rubber stamp, such as APPROVED, DRAFT or CONFIDENTIAL. It can be a standard stamp or a custom image, such as a logo or signature. Stamps show a document's status at a glance. As annotations they can be moved or removed, unless the PDF is flattened.

Also known as: password security handler

The standard security handler is the built-in way PDFs are protected with passwords, rather than with certificates. It turns the user or owner password into an encryption key, encrypts the file with RC4 or AES and stores the permission settings. Every compliant PDF reader supports it, which is why it is the normal choice for protecting files you share.

Also known as: PDF filter, Filter, stream encoding

A stream filter is the method used to encode data inside a PDF, named in the stream's Filter entry. Common examples are FlateDecode for general compression, DCTDecode for JPEG images and ASCII85Decode for text-safe encoding. Several filters can be chained. Compressing a PDF often works by switching to better filters or settings, which reduces size without changing the content.

Also known as: strikethrough, strike-through

A strikeout annotation draws a line through selected text to suggest it should be deleted. It only marks the text, so the words stay in the document until someone actually edits them. Reviewers often pair a strikeout with a note explaining the change. It is a common tool when proofreading or reviewing contracts in PDF.

Also known as: structure elements, StructElem, structural element

A structure element is one item in a tagged PDF's structure tree, such as a heading (H1), paragraph (P), table or figure. Elements nest inside each other to form the document's outline, and each one links to the content it describes on the page. Correct elements let screen readers tell users what each part of the page is.

Also known as: TOC, contents page

A table of contents (TOC) is a list of a document's sections and their page numbers, usually near the front. In PDFs, the entries are often links that jump straight to each section. It gives readers an overview and quick access to content. It works together with bookmarks, which show the same structure in the viewer's side panel.

Also known as: accessible tables, table structure tags, TH TD tags

Table tags are the tags that describe a table in a tagged PDF: Table, TR for rows, TH for header cells and TD for data cells. When headers are linked to their cells, a screen reader can announce the right row and column header as the user moves through the table. Without correct table tags, the data is read as a jumble of values.

Also known as: TIFF, TIF

TIFF is an image format that supports lossless compression, multiple pages, high bit depth and CMYK color. It is common in scanning, faxing and printing. Multi-page TIFF scans are often converted into one searchable PDF. Some print shops also ask for PDF pages exported as TIFF images. TIFF files are usually much larger than the same pages as a compressed PDF.

Also known as: PDF tags, structure tags

A tagged PDF is a PDF with a hidden structure that marks its headings, paragraphs, lists, tables, images and reading order. Tags are defined in the PDF standard and required by PDF/UA. They let screen readers read the file properly, let phones reflow the text and help converters keep the structure. An untagged PDF can look fine but be unusable for assistive technology.

Also known as: sticky note, note annotation, comment note

A text annotation, usually called a sticky note, is a comment pinned to a point on a PDF page. It shows as a small icon, and clicking it opens a pop-up with the comment. Because only the icon sits on the page, reviewers can leave feedback without covering the content. Sticky notes can be listed, replied to and removed later.

Also known as: text box, text input field

A text field is a PDF form field where you type your own answer, such as a name, address or comment. It can be single-line or multi-line, or hide what you type like a password box. It can also limit length or format numbers and dates. Text fields are the most common way PDF forms collect answers.

Also known as: OCR layer, hidden text

A text layer is invisible text that OCR adds to a scanned PDF page, with each word placed exactly where it appears in the image. You still see the original scan, but you can now select, copy and search the words in any PDF reader. Text layers are also what screen readers and converters use to read a scanned document.

Also known as: 3D Annotation, U3D, PRC

A 3D annotation is an interactive three-dimensional model embedded in a PDF page, usually in U3D or PRC format. Readers can rotate it, zoom in and look at individual parts. It is used in engineering and product manuals. Only some viewers, mainly Adobe Acrobat and Reader, can display it, and the model data can make the file much larger.

Also known as: TSA, trusted timestamp, timestamp, RFC 3161

A Time Stamping Authority (TSA) is a trusted service that issues digital timestamps proving that something existed at a certain moment. Following RFC 3161, the TSA signs a fingerprint of the signature together with the time. Adding a TSA timestamp to a signed PDF proves when it was signed, and helps the signature stay valid after the signer's certificate expires.

Also known as: ToUnicode, ToUnicode mapping

A ToUnicode CMap is a table inside a PDF that tells viewers which real characters each glyph in a font stands for. It is what makes text selectable, copyable and searchable. When it is missing or wrong, copied text comes out as random symbols even though the page looks fine. Running OCR on the pages is often the only way to fix that.

Also known as: pdf trailer, file trailer

The trailer is a small section at the end of a PDF file that tells a reader where to start. It points to the document catalog, the number of objects, the file ID and, for edited files, the previous trailer. A PDF reader reads the end of the file first, finds the trailer and follows it to the rest of the content.

Also known as: Flattening, Flatten Transparency

Transparency flattening turns see-through effects in a PDF, such as drop shadows, soft edges, blend modes and opacity, into solid shapes and images that look the same. Some older printers and print workflows cannot handle live transparency, so flattening makes output predictable. It cannot be undone, and it can turn text into images or leave thin lines between flattened areas.

Also known as: ink trapping

Trapping is a print technique where neighboring colors are made to overlap very slightly. Printing plates never line up perfectly, and without trapping thin white gaps can appear where two colors meet. Trapping is applied by design software or by the printer's RIP. It matters most in multi-color jobs on fast presses, where paper stretch makes small shifts likely.

Also known as: trim box

The TrimBox is a page box that marks the final size of a printed page after it has been cut. Anything outside it, such as bleed or printer's marks, is trimmed away. If it is not set, it matches the CropBox. Printers and imposition software use it to position pages correctly, so print-ready PDFs should set it to the finished size, such as A4.

Also known as: TrueType, TTF

TrueType is a font format created by Apple and later licensed to Microsoft, using the .ttf extension. It includes hinting instructions that keep text sharp on screens at small sizes. TrueType fonts are among the most common fonts embedded in PDFs. How the font is embedded and mapped decides whether text in the PDF displays, prints and copies correctly.

Also known as: Technology Without An Interesting Name, TWAIN driver, TWAIN scanner

TWAIN is a software standard that lets programs get images directly from scanners and cameras. Its name is not an official acronym, though 'Technology Without An Interesting Name' is a popular joke. When you scan paper to PDF on a computer, the TWAIN driver controls settings such as resolution, color mode and double-sided scanning before the pages are saved.

Also known as: underline

An underline annotation draws a line under selected text to draw attention to it without covering the words. Like a highlight, it stores the position of the covered text and a color. It is a lighter way to mark terms or references than highlighting. It sits on top of the page, so you can remove it later without changing the text.

Also known as: open password, document open password

A user password, also called an open password, is the password you must enter to open an encrypted PDF. Without it, the file cannot be viewed at all. This is the main protection for confidential PDFs, unlike an owner password, which only limits actions. Share the password through a different channel than the file, such as a phone call or text message.

Also known as: user coordinate space, default user space

User space is the coordinate system used to describe everything on a PDF page before it is shown on a screen or printed. By default it starts at the bottom-left corner, with one unit equal to one point (1/72 inch). Placing text and images, scaling and rotating all happen in user space, which is why a PDF looks the same on any device.

Also known as: vera PDF

veraPDF is a free, open-source tool that checks whether a file really meets the PDF/A or PDF/UA standards. It was built by the PDF Association and the Open Preservation Foundation. It turns each rule in the standards into an automatic test and reports every failure. Archives, governments and publishers use it to prove a PDF meets the level it claims.

Also known as: PDF printer, print driver

A virtual printer is a printer driver that appears in your list of printers but saves a file, usually a PDF, instead of printing on paper. You can choose it from any program's print dialog. It is how Print to PDF works, and it lets programs without a PDF export option still create a PDF that keeps their layout.

Also known as: watermarking, watermark PDF

A watermark is text or an image placed across a page, such as CONFIDENTIAL, DRAFT or a company logo, to show status or ownership. It is usually faint so the content stays readable. In PDFs, watermarks discourage copying, mark draft versions and brand every page. A watermark added as page content is harder to remove than one added as an annotation.

Also known as: WCAG, WCAG 2.1, WCAG 2.2

WCAG (Web Content Accessibility Guidelines) is the W3C's set of rules for making digital content usable by people with disabilities. It was written for websites but is also applied to PDFs, covering alt text, reading order, color contrast and structure. Many laws and organizations require WCAG Level AA. For PDFs, meeting it usually starts with proper tagging.

Also known as: widget, form widget

A widget annotation is the visible part of a PDF form field, the box you see and click on the page. It sets the field's position, border, background and appearance. One field can have several widgets, for example a name repeated on every page, and all of them share one value. The field holds the data, and the widget shows it.

Also known as: X.509, x509 certificate, digital certificate

An X.509 certificate is the standard format for digital certificates. It links a public key to a person or organization and includes the owner's name, the issuer, validity dates and the issuer's signature. Digital signatures in PDFs use X.509 certificates. When you verify a signed PDF, the viewer checks the certificate, its chain of trust and whether it has been revoked.

Also known as: XFA, XFA form, dynamic XFA form

XFA (XML Forms Architecture) is an Adobe form technology that stores a form's layout and logic as XML inside a PDF. This allows forms that grow as you fill them. XFA was deprecated in PDF 2.0, and most browsers and phone apps cannot display it. If a form only shows a 'please wait' message, it is probably XFA and needs Adobe Reader or conversion to a normal form.

Also known as: XFDF, XML Forms Data Format, .xfdf

XFDF (XML Forms Data Format) is the XML version of FDF. It stores the values typed into a PDF form, and can also hold comments and other annotations, as readable XML. Because it is plain XML, it is easy to create and read with standard tools. Apps use it to move form answers or comments between copies of a PDF.

Also known as: XPS, OpenXPS, OXPS

XPS (XML Paper Specification) is a Microsoft document format with fixed pages, created as an alternative to PDF. Like PDF, it keeps the page layout the same on any device. It shows up as files from Microsoft XPS Document Writer, which is used for Windows print jobs. PDF is far more widely supported, so XPS files are usually converted to PDF.

Also known as: XMP, Adobe XMP, XMP metadata

XMP (Extensible Metadata Platform) is a metadata format created by Adobe and now an ISO standard. It stores information about a file as XML inside the file itself. In a PDF it lives in a metadata stream and can hold standard fields, such as Dublin Core, plus custom ones. It keeps metadata more reliably than the older Info dictionary when files pass between tools.

Also known as: fit to page, fit width, magnification

Zoom sets how large a PDF page appears on screen. Fit options scale the page to the window automatically, either fit page, fit width or fit height. On phones, fit width is usually easiest to read. Authors can store a preferred zoom in the PDF's initial view or in links, though many viewers open at their own default.