Alternate and mix PDF pages — the duplex scanning fix
Most home and office scanners with a sheet feeder only scan one side at a time. To digitise double-sided pages you scan the stack once for the fronts, flip it, and scan again for the backs — and end up with two PDFs, or one PDF with all the odd pages followed by all the even pages, often with the backs in reverse order. The Alternate & Mix PDF tool puts them back together: front 1, back 1, front 2, back 2, and so on.
Add the fronts PDF and the backs PDF, tick “Reversed” for a file whose pages came out last-to-first, and check the coloured preview strip before saving. You can also mix three or more PDFs in turn, take several pages from each file per turn, and decide what happens when one file has fewer pages.
Everything happens in your browser — nothing is uploaded. Pages are copied, not re-rendered, so text and images keep their full quality.
How to interleave fronts and backs
- Under “What do you have?”, choose “Two or more PDFs” — or “One PDF: odd pages, then even” if both passes are in a single file.
- Drop the fronts PDF first, then the backs PDF (or both at once in that order). Use the arrows to change the order.
- Leave “Reversed” ticked on the backs if you flipped the whole stack before the second pass; untick it if the backs are already in order.
- Check the preview: tiles marked A1, B1, A2, B2… should read in the order of the paper document.
- Press “Mix pages”, then “Download PDF”.
Alternate & Mix PDF features
Built for duplex scans
Fronts and backs are interleaved page by page, with the backs read backwards when the stack was flipped.
One-file mode
A single scan containing all odd pages followed by all even pages is split at the point you choose and interleaved.
More than two files
Mix three or more PDFs in turn — A1, B1, C1, A2, B2, C2 — and take 1 to 50 pages from each per turn.
Unequal page counts handled
Add leftover pages at the end, insert blank pages to keep every front paired with a back, or stop at the shortest file.
Colour-coded preview
Every tile shows which file and page it comes from, so mistakes are obvious before you save.
Lossless
Pages are copied exactly; the first file’s bookmarks and document details are kept.
Why the backs are often reversed
After the first pass, the scanned sheets come out of the feeder in a pile. Turning that whole pile over to scan the other side puts the last sheet on top, so the second pass starts with the back of the last page. With 5 double-sided sheets, the backs PDF holds pages 10, 8, 6, 4, 2 in that order. Ticking “Reversed” reads it backwards so each back lands behind its front.
| Pass | Pages as scanned | Setting |
|---|---|---|
| Fronts (file A) | 1, 3, 5, 7, 9 | Reversed off |
| Backs after flipping the stack (file B) | 10, 8, 6, 4, 2 | Reversed on |
| Backs after re-sorting the sheets (file B) | 2, 4, 6, 8, 10 | Reversed off |
| Result | 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 | — |
When the page counts don’t match
- Add the other pages at the end — the default. Mixing continues with the files that still have pages, so nothing is lost.
- Insert blank pages to keep pairs — when a sheet’s back was blank and the scanner skipped it, a blank page keeps every following front and back aligned for double-sided printing.
- Stop when the shortest file ends — useful when the longer file contains extra pages you don’t want; the preview tells you how many are left out.
Related tools
To join files one after another instead of page by page, use Merge PDF. A single file that only needs flipping can be fixed with Reverse PDF, individual pages moved with Organize PDF, and a missing blank back added with Insert PDF Pages.
Alternate & Mix PDF FAQ
How do I merge odd and even pages into one PDF?
If they are in two files, choose “Two or more PDFs”, add the odd pages first and the even pages second, and press “Mix pages”. If both are in one file, choose “One PDF: odd pages, then even” and enter the page where the even pages start.
How do I combine front and back scans into one PDF?
Add the fronts PDF, then the backs PDF. If you flipped the stack for the second pass, keep “Reversed” ticked on the backs. The preview shows A1, B1, A2, B2… — then press “Mix pages”.
What if one side has fewer pages?
Choose what happens in “When a file runs out of pages”: add the remaining pages at the end, insert blank pages to keep fronts and backs paired, or stop when the shorter file ends.
Can I alternate pages from more than two PDFs?
Yes. Add up to 10 PDFs; pages are taken from each in turn (A, B, C, A, B, C…), and you can take several pages per turn.
Does mixing PDFs reduce quality?
No. Pages are copied exactly as they are, without re-rendering or recompression.
Is it safe to mix confidential scans here?
Yes. All processing happens in your browser and the files are never uploaded.