How to use it
- Choose two files (fronts and backs) or one file (all fronts, then all backs).
- Drop the PDF or PDFs.
- Leave Back pages are in reverse order on if you flipped the stack.
- Click Merge odd and even pages.
The single-sided scanner problem
Many office scanners and home all-in-ones with a document feeder scan only one side of each sheet. To scan a double-sided document you run the stack through twice: once for the fronts (pages 1, 3, 5…) and once, after flipping the stack, for the backs (pages 2, 4, 6…). The result is two PDFs, or one PDF with all fronts followed by all backs, and the backs are usually in reverse order.
Putting that back together by hand in a page organizer is tedious and error-prone with more than a few sheets. This tool interleaves the pages automatically: front 1, back 1, front 2, back 2, and so on.
Two ways to start
| Input | When to use it |
|---|---|
| Two files: fronts + backs | Your scanner saved each pass as its own PDF |
| One file: all fronts first, then all backs | Both passes ended up in a single PDF |
With one file, the first half of the pages is taken as fronts and the second half as backs. If the document has an odd number of pages, there is one more front than back (the last sheet has a blank back that you may have skipped), and that works too. The number of fronts must equal the number of backs, or be one more; otherwise the tool explains the mismatch.
Reversed back pages
When you take the stack of scanned sheets out of the output tray and put it back into the feeder upside down to scan the backs, the last sheet is scanned first. So the backs come out as pages 10, 8, 6, 4, 2 instead of 2, 4, 6, 8, 10.
Back pages are in reverse order is on by default because that is the common case. If your backs were scanned in normal order (for example, you re-sorted the stack, or your scanner software reversed them already), turn it off. Check the preview of the first six pages of the result: if page 2 shows the back of the last sheet, flip the setting and run it again.
Quality and privacy
Pages are copied, not re-rendered, so scan quality is unchanged and any existing text layer stays searchable. Both files are processed in your browser and never uploaded, which matters for the kinds of documents people scan: contracts, medical records, tax documents and school paperwork.
Your original scans are not modified; the merged copy downloads as a new file with -collated added to its name, and you can rerun the merge with different settings as often as you like.
A worked example
You have a 7-sheet double-sided contract (14 pages). Pass 1 through the feeder gives fronts.pdf with 7 pages. You flip the stack, feed it again and get backs.pdf with 7 pages, last sheet first. Choose Two files, drop fronts.pdf in the front slot and backs.pdf in the back slot, leave reverse order on, and click Merge odd and even pages. The result is a 14-page PDF in reading order. If blank backs came through, remove them with Delete PDF pages, then make the contract searchable with OCR PDF.
Frequently asked questions
How do I combine front and back scans into one PDF?
Choose Two files, drop the fronts PDF and the backs PDF, and click Merge odd and even pages.
What if all pages are in one PDF?
Choose One file. The first half is treated as fronts and the second half as backs.
Why are my pages in the wrong order?
The backs are probably not reversed (or are). Toggle Back pages are in reverse order and merge again.
What if there's one more front than back?
That's fine. The last front page is placed at the end.
Is my scan uploaded?
No. The pages are combined in your browser.
Last updated: November 12, 2026