Sheet-fed scanners without a duplex unit are still everywhere, and they all create the same problem. You feed the stack to capture the fronts, turn the whole pile over, feed it again to capture the backs — and now you hold two PDFs that need to interleave rather than concatenate, with the second one in reverse order.
How to merge front and back scans
- Scan the fronts into one PDF, then turn the whole stack over and scan the backs into a second.
- Open ihatepdf.cv/alternate-mix-pdf and add both files, fronts first.
- Switch on Reverse for the backs, because turning a stack over reverses it.
- Check the order preview — it should read A1 Bn A2 Bn-1, fronts forwards and backs backwards.
- Interleave and download. The pages come out in reading order.
Why the backs need reversing
Turning a stack of paper over reverses it. The sheet that was on top is now on the bottom, so the feeder captures the back of the last page first. Reversing that file undoes the flip.
This is not universal, which is why it should be a toggle rather than something applied automatically. If your scanner ejects face-up rather than face-down, or if you turn the stack end-over-end rather than side-over-side, the backs may already be in order. The order preview is how you tell without printing anything.
Interleaving is not merging
Merging concatenates: the whole of the first file, then the whole of the second. You get all the fronts followed by all the backs, which is not a document anybody can read. Interleaving alternates page by page, which is what parallel documents need.
The distinction matters because merge is the tool everyone reaches for first, and it produces a plausible-looking file with the right page count and the wrong order.
Check the order before you commit
The failure mode here is not a crash. It is a document that looks fine and is wrong — right page count, wrong sequence, discovered weeks later by whoever has to read it.
That is why seeing the resulting order as an explicit sequence matters. A1 B3 A2 B2
A3 B1 tells you at a glance that file A runs forwards and file B backwards, which is
exactly what a duplex scan should look like. If it reads A1 B1 A2 B2 you have
forgotten the Reverse toggle — and you can see that in a second rather than after printing
forty pages.
What if the counts do not match?
Unequal page counts are normal and should be handled gracefully: when one file runs out the others carry on, so the remaining pages are appended in order. With a duplex scan an uneven count usually means the final sheet was blank on the back and the scanner dropped it, or that blank-page detection removed some backs. Check the tail of the document if the difference is more than one or two.
Blank backs
If you scanned every back including the blank ones, you will end up with a document that alternates content and empty pages. That is easily fixed afterwards with Organize Pages, which lets you delete the blanks in a batch. Some scanners can drop blanks during capture, but they are notoriously unreliable at it on lightly-marked pages, so it is often safer to scan everything and clean up after.
The four names for this tool
If you have been searching for this and found nothing, the naming is probably why. Sejda calls it Alternate & Mix. PDF24 and several others call it Alternate Merge. PDFsam calls it Mix. Plenty of documentation just says interleave, and scanner manuals sometimes say collate. They all describe the same operation: take one page from each file in turn.
Other things interleaving is good for
Beyond duplex scanning, alternating pages builds a bilingual document with each translated page facing its original, assembles a comparison set with each revised page next to the version it replaced, reunites forms with their continuation sheets, or lays out a deck so each slide is followed by its notes page. Anywhere two documents are parallel rather than sequential, interleaving is the operation you want.
Frequently asked questions
How do I merge odd and even pages of a PDF?
Add the odd-page file and the even-page file, reverse the second if it was scanned back to front, and interleave. One page is taken from each in turn.
Why are my back pages in reverse order?
Because turning the stack over reverses it — the feeder captures the back of the last sheet first. Switching on Reverse for that file undoes it.
What if my two files have different page counts?
When one runs out the other carries on and the remaining pages are appended. With a duplex scan this usually means the last sheet was blank on the back.
Can I interleave more than two files?
Yes. One page is taken from each in the order you arrange them, so three files produce A1 B1 C1 A2 B2 C2 and so on.
How is this different from merging?
Merging puts one document after another. Interleaving alternates between them page by page. If you want the whole of file A followed by the whole of file B, use Merge PDF.
Does interleaving re-compress my scans?
No. Pages are copied across intact, so scan quality, OCR text layers and file size are all unaffected.
How do I remove the blank backs afterwards?
Use Organize Pages to delete them in a batch before saving.