Output order: A1, B1, A2, B2, … — perfect for double-sided scans made in two passes.
Files never leave your device.How it works
How to alternate and merge
- 1Add the first PDF (the fronts / odd pages).
- 2Add the second PDF (the backs / even pages); tick reverse if they were scanned last-first.
- 3Click Merge, then download the interleaved PDF.
Frequently asked questions
- When do I need this?
- When a simplex scanner made two files — all the fronts, then all the backs — and you need them woven back into the correct front/back order.
- Why the reverse option?
- Backs often come out of the feeder in reverse order (last page first). Ticking reverse flips the second file so pages line up.
- Does my file get uploaded?
- No. Both PDFs are merged entirely in your browser — nothing leaves your device.
Rebuild double-sided scans
Without a duplex scanner, the usual trick is to scan all the fronts, flip the stack, and scan all the backs — leaving two files that need weaving together. Alternate & Merge interleaves them page by page (A1, B1, A2, B2…) so the document reads in the right order again.
Handling the flip
When you scan the backs, the feeder often takes them last-page-first, so the second file is in reverse. Tick the reverse option and it's flipped before interleaving, lining every back up with its front. If the two files have different lengths, the extra pages are appended at the end.
Notes and limits
Both files are processed locally, so nothing is uploaded. To simply join files end to end, use Merge instead.
How the interleaving works
The tool zips the two files together by index: page 1 of the fronts, page 1 of the backs, page 2 of the fronts, page 2 of the backs, and so on, building an entirely new page order rather than modifying either source file. It's a straightforward index-based merge, which is why it only takes a moment even on a long document.
Edge cases worth knowing
- If the two files have different page sizes, say the fronts were scanned at one DPI and the backs at another, each page keeps its own original dimensions; they aren't forced to match.
- Unequal page counts don't cause an error: once the shorter file runs out, the remaining pages of the longer one are appended in order at the end rather than dropped.
Where this comes up
Beyond the classic simplex-scanner situation, this is also useful for stapled paperwork scanned in two passes by hand, or for combining odd-numbered and even-numbered pages exported separately from another tool. If the reverse checkbox alone doesn't fix a stubborn ordering problem, say one file is reversed and the other isn't quite, running Reverse Pages on the problem file first, then feeding both into Alternate & Merge, gives more control than the built-in option alone.