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How-to

How to rebuild a double-sided scan from two single-sided PDFs

A flatbed scanner or a simplex-only feeder cannot scan both sides of a page in one pass. The usual workaround is to scan every front page first, then flip the whole stack over and scan every back page as a second file. The two resulting PDFs then need to be woven back together, front, back, front, back, in the original page order. The PDF Alternate & Mix tool does that interleaving directly in your browser, including a reverse option for the flipped stack.

Written by Bastien Sulyan · Published

Step by step

  1. Scan the front side of every page into one PDF (document A), then flip the entire stack over and scan every back side into a second PDF (document B). Open the PDF Alternate & Mix tool and load document A in the first file field and document B in the second.
  2. Check Reverse document B if your scanner produced the back pages in reverse order, which is the normal outcome of flipping a stack of paper: the sheet that was on the bottom of the front stack ends up on top once you flip it, so its scan comes out first instead of last. Leave the box unchecked if your scanner or feeder already restores the original order for you.
  3. Name the output file and click Run. Download the merged PDF and open it to check the page order: front 1, back 1, front 2, back 2, and so on. If the pages come out in the wrong order, toggle Reverse document B and run the tool again. There is no live preview before the merge, so this quick check-and-redo is the fastest way to get it right.

Why flipping a stack reverses its page order

Picture a 10-page double-sided document. You scan the fronts in order, page 1 through page 10, keeping the stack face-up. To scan the backs, you flip the whole stack over as one block rather than turning each sheet individually. That single flip reverses the stack: the sheet that was on the bottom, originally page 10, ends up on top, so the scanner reads its back first. The result is a document B where the back of page 10 comes first and the back of page 1 comes last, the exact reverse of document A. Reverse document B undoes that by reading document B's pages back to front before interleaving.

How the interleaving itself works

Once both documents are loaded, the tool builds the output page order by taking page 1 of A, then page 1 of B (or its reversed equivalent), then page 2 of A, then page 2 of B, and so on. If one document has more pages than the other, for example when a blank last page was skipped on one side, the extra pages from the longer document are appended at the end instead of causing an error, so you get a complete document to inspect rather than a failed run.

The scans stay on your device

Both PDFs are loaded and merged with pdf-lib running inside your browser tab; the interleaved result is built in memory and offered as a direct download. Uploading two scanned documents that may contain signatures, ID numbers, or account details to a remote converter is exactly the kind of exposure this workflow avoids: nothing about either file is sent anywhere during the process.

The tools used in this guide

Frequently asked questions

My scanner already puts the back pages in the right order. Do I still need to check Reverse document B?

No. Some scanners and multi-function printers detect the flip and restore the natural order automatically. Whether yours does is a five-second check: run the merge once with the box unchecked, look at the result, and toggle it only if the back pages come out reversed.

What happens if my front and back scans have a different number of pages?

The tool interleaves up to the shorter count, then appends the remaining pages from the longer document at the end rather than stopping with an error. This is common when a trailing blank back page gets skipped during scanning. Always open the merged result and check the page count and order before relying on it, since a mismatch usually means a page was missed during scanning.