Interleave PDF — Merge Front and Back Scans
Browser tool that alternates the pages of an odd-pages (fronts) PDF and an even-pages (backs) PDF
Runs in your browser · nothing is uploaded
Drop the fronts (odd pages) PDF here
The scan of the front sides only: pages 1, 3, 5… (up to 100 MB).
Drop the backs (even pages) PDF here
The scan of the back sides only: pages 2, 4, 6… (up to 100 MB). Files never leave your browser.
If you flipped the whole stack to scan the backs, the back of the last sheet comes out first. That is the case for most sheet-fed scanners.
What it does
Scanning a double-sided document on a single-sided scanner gives you one PDF with the fronts and another with the backs. Simply joining them gives 1, 3, 5, …, 2, 4, 6. This tool takes one page from each file in turn and builds a single PDF in the original 1, 2, 3, 4… order. A reverse option handles backs that came out in reverse, and the merged order is spelled out before you start. Everything runs in your browser.
How to use
- Add the fronts (odd pages) PDF to the first box.
- Add the backs (even pages) PDF to the second box. If you mixed them up, press Swap fronts and backs.
- If you flipped the stack to scan the backs, leave Insert the backs in reverse order on (the default).
- Check the merged order preview (for example F1 · B5 · F2 · B4 …).
- Press Interleave and download the result.
How it works
- Order: front 1, back 1, front 2, back 2, and so on. A reversed file is read from its last page. The fronts can be reversed too.
- Different page counts: one page fewer on the backs is noted; other differences are flagged. Pages without a partner are appended in their order.
- What is copied: page content, size and rotation. Bookmarks and document properties are not included. The file is named after the fronts file with
-interleavedadded. - Orientation: if the backs came out upside down, rotate the even pages by 180° in Rotate PDF afterwards.
- Limits: up to 100 MB per file. PDFs that need a password to open, or that carry editing restrictions, cannot be used.
- Libraries: pdf-lib (MIT) writes the file and PDF.js (Apache-2.0) draws the previews.
Examples
| Fronts file | Backs file | Reverse | Merged |
|---|---|---|---|
| pages 1, 3, 5 | 6, 4, 2 (stack flipped) | on | 1, 2, 3, 4, 5, 6 |
| pages 1, 3, 5 | 2, 4, 6 (scanned in order) | off | 1, 2, 3, 4, 5, 6 |
| pages 1, 3, 5 | 4, 2 (last back blank) | on | 1, 2, 3, 4, 5 |
To simply join files end to end, use Merge PDF; to move or delete pages afterwards, use Organize PDF.
FAQ
Are my PDFs uploaded?
No. Merging runs in your browser only and neither file is uploaded.
When do I need this?
When your scanner or copier cannot scan both sides automatically. You scan the fronts of the stack once, flip the stack and scan the backs, and end up with two PDFs. This tool puts their pages back in the original order.
What does reversing the backs mean?
If you flip the whole stack to scan the backs, the back of the last sheet is scanned first, so the backs PDF runs 10, 8, 6… With "Insert the backs in reverse order" on, pages are taken from the end of that file, giving 1, 2, 3, 4… It is on by default; turn it off if you scanned the backs from the first sheet.
The two files have different page counts.
If the last back side was blank and not scanned, having one page fewer on the backs is normal and the files are merged as they are. Larger differences show a warning, and pages without a partner are appended at the end.
I added the files the wrong way round.
Press "Swap fronts and backs" to switch their roles without adding them again.
Related tools
Merge PDF
Browser-based PDF merger with previews and drag-to-reorder, no upload
Split PDF Pages in Half
Browser tool that cuts each PDF page into two halves, left/right or top/bottom
Organize PDF
Reorder, rotate and delete PDF pages with thumbnails, in your browser
Rotate PDF
Browser tool that rotates all or selected PDF pages and mirrors them horizontally or vertically