It is one of the most common paper-handling headaches in any office. You have a stack of double-sided pages and a scanner that only reads one side at a time. So you scan all the fronts into one PDF, flip the stack, scan all the backs into a second PDF, and now you are holding two files that need to become one, with the pages in the correct front-back-front-back order. Retyping is not an option, and rescanning page by page is miserable. This is exactly the problem interleaving solves.
Why the two files fight each other
When you scan the fronts, you get pages 1, 3, 5, 7 in order. When you flip the whole stack over and feed it back through, the scanner reads the last physical sheet first, so the backs usually come out reversed: page 8, 6, 4, 2. Simply appending the second file to the first gives you every odd page followed by every even page in the wrong direction. The content is all there, but the reading order is scrambled, and fixing it by hand means dragging dozens of pages around one at a time. The pattern is predictable enough that a tool can rebuild it automatically once it knows which file is which.
How alternate mix reassembles the document
The Alternate Mix tool is built for precisely this. You give it two PDFs, and it interleaves them: it takes the first page of file one, then the first page of file two, then the second page of file one, then the second page of file two, and so on. That woven order is exactly how a double-sided document reads: front, back, front, back. Crucially, it includes an option to reverse the second file, which handles the flipped-stack problem automatically so your backs line up with the right fronts instead of running backwards.
- Scan all the fronts into one PDF and all the backs into a second PDF, without reordering either stack.
- Open the Alternate Mix tool and select the fronts file first and the backs file second.
- Turn on the reverse-second-file option so the backs, which came out in reverse, are flipped to match the fronts.
- Generate the interleaved PDF and skim through it to confirm each back follows its correct front.
- If the order still looks off, try it once with the reverse option turned off, since some scanners and manual flips produce the backs already in forward order.
The reason for that last step is simple: whether the backs come out reversed depends on how you flipped the stack and how your scanner feeds paper. Rather than guess, the fastest test is to try both settings and glance at the first few page pairs. The correct combination is obvious within seconds, because a mismatch shows a back that clearly belongs to a different front. Once you find the setting that works for your scanner, it stays the same for every future double-sided job you run the same way.
Cleaning up after the merge
Interleaving fixes the order, but scans often need a little tidying afterward. A blank back on the final sheet, a stray page fed twice, or a cover sheet you no longer want can all be handled without rescanning. Use the Organize tool to drag, remove, or reorder any pages that slipped out of place, and to delete blank backs the scanner captured. Then run the OCR tool over the finished document so the scanned images gain a real, searchable text layer, turning pictures of pages into text you can find and select. OCR is worth the extra step for any scan you will keep: straight out of the scanner each page is just an image, so searching finds nothing and you cannot copy a sentence out of it, whereas OCR analyzes the pixels and adds an invisible text layer underneath, making the reassembled document behave like a born-digital PDF. Skim the interleaved result once more end to end, since a single missing or duplicated sheet shifts every page after it.
Getting cleaner scans in the first place
A little care at the scanner saves cleanup later. Keep the front stack and the back stack in the same physical orientation, avoid reshuffling either pile between passes, and note which file is which so you do not swap the order in the tool. If your pages are landscape or came in rotated, the Rotate tool can straighten them, and it is easiest to do that before interleaving so both files share the same orientation. Because everything runs in your browser, even sensitive scans like contracts or medical forms stay on your own device throughout the whole reassembly, from the two raw files to the finished, searchable document. Once you have a routine that works for your scanner, the whole process takes a couple of minutes and reliably turns two awkward half-documents back into the single double-sided file you started with, no manual page-dragging and no trip back to the scanner required.