Scanned the fronts of a double-sided document, then the backs? Interleave the two PDFs page by page into one correctly ordered file. Reverse the back sides with one tick and check the page order before you save. Files never leave your device.
Last updated:
A Fronts / odd pages
B Backs / even pages
Merged page order File A File B
Add your two PDFs to see the merged page order.
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How to use the Alternate Merge PDF
Pick a mode: Two files for separate front and back scans, or One file when a single scan contains all fronts followed by all backs.
Add the PDFs: the fronts (odd pages) as File A and the backs (even pages) as File B. Use Swap if you added them the wrong way round.
Tick Reverse File B if you turned the stack over and scanned the backs last page first. That is the usual case with a single-sided feeder.
Check the page order preview. Each chip shows which file and page lands in each position of the merged PDF.
Press Merge alternately to download the combined PDF.
Why scanned double-sided pages come out of order
Many home and office scanners have a feeder that scans only one side of each sheet. The usual workaround is to scan the stack once for the front sides, then turn the whole stack over and scan again for the back sides. You end up with two files: the fronts contain pages 1, 3, 5, 7 and the backs contain pages 8, 6, 4, 2, because turning the stack over also reverses its order. Alternate merging with File B reversed rebuilds the original sequence.
Scenario (4 sheets)
File A
File B
Settings
Result
Stack flipped over for the backs
1, 3, 5, 7
8, 6, 4, 2
Reverse File B
1, 2, 3, 4, 5, 6, 7, 8
Backs re-fed in the same order
1, 3, 5, 7
2, 4, 6, 8
No reversing
1, 2, 3, 4, 5, 6, 7, 8
One scan, fronts then flipped backs
1, 3, 5, 7, 8, 6, 4, 2 in one file
One file mode, backs reversed
1, 2, 3, 4, 5, 6, 7, 8
Insert a sheet after every two pages
Report (10 pages)
Separators (5 pages)
2 from A, 1 from B
A1, A2, B1, A3, A4, B2…
Tips for a clean result
Count the pages first. Both scans of a double-sided stack should have the same number of pages. If they differ, a sheet was probably double-fed or a blank back was dropped by the scanner's blank page removal. Turn that feature off for duplex work.
Check the first chips. The order preview shows exactly which source page goes where. For a flipped stack the second chip should be the last page of File B.
Fix orientation afterwards. If the backs were scanned upside down, open the merged file in the Rotate PDF tool and rotate the even pages by 180°.
Remove blank backs later. Single-sided sheets in a duplex scan leave empty pages; delete them once the order is right so numbering stays consistent.
Frequently asked questions
How do I combine odd and even pages scanned separately?
Scan all front sides into one PDF (pages 1, 3, 5…), flip the stack and scan the back sides into a second PDF. Add the fronts as File A and the backs as File B, tick Reverse File B if the backs came out last page first, and press Merge alternately. The result reads 1, 2, 3, 4 in order.
Why do the back pages need to be reversed?
When you flip a stack of paper over to scan the other side, the last sheet is now on top, so the backs are scanned from the last page to the first. Reversing File B puts them back in order before interleaving. If your scanner kept the original order, leave the box unticked.
What happens if the two files have a different number of pages?
Pages are interleaved until the shorter file runs out, then the remaining pages of the longer file are appended at the end. The tool shows a warning, because a difference usually means a sheet was skipped or a blank back side was removed during scanning.
Can I interleave pages from a single PDF?
Yes. Choose One file mode for a scan that contains all fronts followed by all backs. The tool splits it at the middle (you can change the split point) and interleaves the two halves, with the option to reverse the backs.
What does pages per turn do?
It sets how many consecutive pages are taken from each file before switching. With 2 from A and 1 from B the order is A1, A2, B1, A3, A4, B2. This is handy for inserting a separator page after every pair or alternating chapters.
Are my documents uploaded?
No. Both PDFs are read and merged in your browser with the pdf-lib library. Nothing is sent to a server, so it is safe for contracts, statements and other private scans.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.