Alternate & Mix Two PDFs
Scanned the fronts in one pass and the backs in another? This zips the two files together — page 1 from the first, page 1 from the second, page 2 from the first, and so on — with a switch for the case where the backs came out in reverse order.
Drop exactly two PDFs here
First file = odd pages · second = even pages
No PDF handy? The sample library has multi-page files to test with.
The duplex-scanner problem
A single-sided sheet feeder gives you two files: odd pages in order, and even pages in whatever order the stack came out. Flip the pile and feed it again and the backs arrive last-to-first, so page 2 of the document is the last page of the second file. That is what reverse it is for — turn it on and the interleave counts the second file backwards.
If your scanner produced the backs in forward order, leave it off. One run tells you which: check whether page 2 of the output is the back of page 1.
Uneven page counts
The files do not have to be the same length. When one runs out, the remaining pages of the other are appended in order — which is exactly right for a document with an odd number of sheets, where the last back is blank and was never scanned.
The stride option
Normally one page is taken from each file in turn. A stride of 2 takes two from the first, then two from the second, and so on — useful when each "unit" is a two-page spread, or when a booklet was scanned in pairs. Anything above 1 is unusual; if you are not sure, leave it at 1.
FAQ
Which file should I add first?
The one with the odd pages — the fronts. The output starts with its first page. The file list shows the order and lets you drag to swap them.
How do I know whether to reverse the second file?
Run it once and look at page 2 of the result: if it is the back of page 1, you are right. If it is the back of the last page, turn reversing on and run again.
What if one file has more pages?
The leftovers are appended at the end in order. Nothing is dropped.
Does it re-compress anything?
No. Pages are copied between documents, so quality, text and links are untouched.
Is my scan uploaded?
No. Both files are read and combined in your browser.
Related tools
- Merge PDFs — plain concatenation
- Reorder pages — fine-tune the order afterwards
- Rotate pages — fix upside-down backs
- Remove blank pages — drop the empty backs
- OCR PDF — make the combined scan searchable