How to Combine Front and Back Scans into One PDF

Many home and office scanners have an automatic document feeder that reads only one side of each sheet. To capture both sides, you feed the stack twice. The first pass reads every front. You then turn the whole stack over and feed it again, and the sheet that came out last now goes in first.

The result is two PDFs. If you join them end to end with a merge tool, every back lands after every front, and in reverse order. You need one page from each file in turn, after the backs file has been reversed. The table shows a stack of 3 sheets at each stage.

Not every feeder behaves the same way. Open the backs PDF and look at its first page. If it is the back of the last sheet, the backs are reversed. If it is the back of the first sheet, they are already in order.

Interleave PDF takes two PDFs, or one PDF that holds every front followed by every back. It copies the pages into a new file instead of drawing them again. Scanned images keep their resolution, an OCR text layer stays searchable, and every page keeps its size and rotation.

The same tool merges odd and even pages kept in two files. This happens when someone exports or prints the odd pages and the even pages separately, or scans every right-hand page of a bound document and then every left-hand page.

Add the odd pages PDF first and the even pages PDF second. Turn Reverse the backs off, since the even pages are already in order. The result runs 1, 2, 3, 4 and so on. If the second file holds page 1, pick Back first instead of swapping the files.

Some scanning apps add the second pass to the end of the same file. You then have one PDF with every front followed by every back. Add only that file to Interleave PDF and enter how many of the first pages are fronts.

The number starts at half the page count, rounded up. A 20-page scan splits into 10 fronts and 10 backs, and a 21-page scan into 11 fronts and 10 backs. Change it when a sheet was skipped or fed twice, so the split matches the page where the backs really begin.

If the document is longer than your feeder holds, scan it in batches. The safest order is to finish each batch before you start the next one. Scan the fronts and backs of batch 1 and interleave them, then do the same for batch 2.

When every batch is interleaved, open Merge PDF, add the batch files, put them in order with the arrows and merge them. Merge PDF adds a bookmark for each file, so each batch is easy to find in the finished document.

You can also merge all the front batches into one file and all the back batches into another. In that case, add the back batches in reverse order, last batch first. When Interleave PDF reverses the whole backs file, every back then lines up with its front. Most people find it easier to interleave batch by batch.

If one set has more pages than the other, Interleave PDF asks what to do with the leftovers. Add the extra pages at the end pairs pages until the shorter set runs out, then adds the rest in order. Insert blank pages gives each unpaired page a blank partner of the same visible size, so the file still prints double-sided with every front backing onto the right page.

A sheet that was skipped or fed twice in the middle of the stack is a different problem. Every pair after it is off by one. The preview labels show where this starts, since a front and a back that do not belong together sit side by side. Rescan the missing side, or move the stray pages by hand in Delete / Reorder, where you drag page thumbnails into a new order.

Interleave PDF keeps every page. If some sheets were printed on one side only, their scanned backs become blank pages in the result. Remove them after interleaving, not before, because deleting pages from the backs file shifts every later pair.

Remove Blank Pages renders each page at up to 60 DPI and measures how much of it is ink. At the default level, a page counts as blank when 0.1% of it or less is ink, so a page with one line of text is kept. Every page appears as a thumbnail, and you can tap any page to keep or remove it before saving.

Keep the blank backs if you plan to print the file double-sided again. Without them, a front can end up on the back of the wrong sheet.

If you scan from inside the Adobe Acrobat desktop app, its scan settings have a Both sides option. Adobe's help says this works on scanners that cannot scan two sides. After the fronts are scanned, Acrobat asks you to turn the stack over and choose Scan reverse sides, and it saves the pages in order. This only helps when Acrobat drives the scanner, not with two PDFs you already have.

On a Mac, Preview can move pages between two open PDFs. Choose View > Thumbnails in both windows, then drag each back from the backs PDF into place under its front. This works for three or four sheets and is slow for thirty.

Most ordering problems start at the scanner. These habits prevent them, and the tools at the end clean up what the scanner leaves behind.