Merge alternate pages to fix a duplex scan
Fix duplex scans by interleaving an odd-page file with an even-page file, reversing the second pass automatically. No upload, no page limit.
- Runs in your browser
- Files are never uploaded
- Free, no account
Both scans never leaves this tab — here is how to check
This page has no upload step and no server behind it. Two ways to confirm that yourself, neither of which requires trusting us:
- Turn off Wi-Fi or put the device in aeroplane mode after this page has loaded, then run the tool. It still works.
- Open your browser’s developer tools, switch to the Network tab, clear it, and process a file. No request appears.
The reason this matters: in 2025 the PDF service pdf.net was found exposing roughly 3.5 million user-uploaded documents, and formatter sites JSONFormatter.org and CodeBeautify leaked over 80,000 saved snippets containing live cloud credentials. Those are architectural failures — a file that is never sent cannot be exposed that way.
The duplex scan problem
Cheap sheet-feed scanners and most multifunction printers only image one side at a time. To scan a double-sided document you feed the stack, get a PDF of the fronts, flip the whole stack over, feed it again, and get a second PDF of the backs. Neither file is usable on its own, and the second one is in reverse order because flipping the stack puts the last sheet on top.
Interleaving them fixes it in one step: front 1, back 1, front 2, back 2, and so on, in a single document with the pages in the order a human would read them.
Worked example
A ten-page, five-sheet document scanned in two passes:
| Output page | Comes from | Which page of that file |
|---|---|---|
| 1 | File A — fronts | 1 |
| 2 | File B — backs, reversed | 5 (was scanned last) |
| 3 | File A | 2 |
| 4 | File B, reversed | 4 |
| 5 | File A | 3 |
| 6 | File B, reversed | 3 |
| … | … | … |
The order preview above the button shows exactly this mapping for your two files before anything is written, so a wrong reverse setting is obvious in a second rather than after you have printed 200 sheets.
When to turn the reverse switch off
- Off if you rotated the stack rather than flipping it, or if your scanner software already reversed the second pass for you.
- Off if the two files come from different sources — for example alternating pages of a translation and its original.
- On — the default — for the ordinary flip-the-stack case.
Not sure? Look at the last row of the preview. If output page 2 draws from the highest page number in file B, the reverse is applied.
Uneven page counts
Almost every real production has them, usually because the final sheet was printed on one side. Keep the default, keep the extra pages at the end, and the surplus pages are appended rather than silently dropped. Choose stop when the shorter file runs out when the backs are genuinely blank and you would rather not carry them.
A mismatch larger than one page usually means a misfeed: two sheets pulled through together on one of the passes. Rescan rather than patch — the preview will show you which pass is short.
Nothing is re-compressed
Pages are copied between documents as complete PDF objects. The scan images keep their original resolution and compression, colour profiles are carried across, and any OCR text layer your scanner produced survives intact and searchable. The output is byte-equivalent page content in a new order, not a re-render.
Related fixes for a bad scan
- Reverse PDF Page Order — when only one file is in the wrong direction.
- Split PDF Pages in Half — when two pages landed on one scan.
- Rotate PDF — when the backs came out upside down.
- Delete Pages from PDF — for the blank backs of single-sided sheets.
Questions
Why does the second file need reversing?
A single-sided sheet feeder scans page 1, 3, 5 on the first pass. When you flip the whole stack over and feed it again, the last sheet goes in first, so the backs come out as page 10, 8, 6, 4, 2. Reversing that file puts them back into ascending order before interleaving.
The two files have different page counts — is that a problem?
Not usually. It normally means the last sheet was printed on one side only. Keep the default “keep the extra pages at the end” and check the order preview: the blank or missing back simply has no counterpart.
Does interleaving re-compress the scans?
No. Pages are copied object-for-object between documents, so the image data, resolution, colour profile and any text layer from OCR are all preserved byte-for-byte.
Can I do this with one file that has all the fronts then all the backs?
Split it into two files first with Split PDF — take pages 1 to N as the fronts and N+1 to the end as the backs — then bring both here. Doing it in two steps keeps the ordering explicit and easy to verify.