Alternate and mix two PDFs
Interleave two PDFs — the fix for scanning double-sided pages in two passes.
Drop your file here
Processed securely and deleted within the hour.
The double-sided scanning problem
A single-sided scanner cannot do double-sided documents in one pass. So you scan all the fronts, flip the stack, and scan all the backs — and end up with two files that merging cannot fix.
Merging gives you pages 1, 3, 5, 7 followed by 2, 4, 6, 8. What you need is 1, 2, 3, 4 — the two files interleaved, one page from each in turn. That is what this does.
Why the second file is reversed by default
Because of how flipping a stack works. Scan pages 1, 3, 5, 7 face down, pick up the pile, turn it over, and feed it again — the backs come out in the order 8, 6, 4, 2, because the last sheet is now on top.
Reversing is therefore the correct default, and it is the setting people most often need and least often think of. If your scanner produced the backs in forward order — some do, and some workflows reverse the stack manually — turn it off.
Checking it worked
Look at pages 2 and 3 of the result. If page 2 is the reverse of page 1 and page 3 is the front of the second sheet, the order is right. If page 2 is the last page of the document, the reverse setting needs flipping.
Uneven page counts
If the two files have different numbers of pages, the interleave still runs and the extra pages appear at the end — but you are warned, because it usually means a sheet was missed or double-fed during one of the passes.
Related
If pages also came out upside down, rotate afterwards. To reorder freely instead, organize.
Common questions
What is this for?
Combining separately scanned fronts and backs of a double-sided document into the correct order.
Why is the second file reversed by default?
Flipping a scanned stack puts the last sheet on top, so the backs come out in reverse. That is the usual case.
How do I check the order is right?
Page 2 of the result should be the reverse of page 1. If it is the last page of the document, flip the reverse setting.
What if the files have different page counts?
It still runs and warns you — usually it means a sheet was missed or double-fed.
Can I do this with more than two files?
Not in one pass. Interleave two, then interleave the result with a third.