You scanned a double-sided document on a scanner that can only do one side at a time. First you scanned the fronts, then you flipped the stack over and scanned the backs. Now you have two PDFs, and what you actually want is one file with page 1 front, page 1 back, page 2 front, page 2 back, and so on. That's exactly what this tool does. It interleaves two PDFs, one page from each in turn, and handles the "backs came out in reverse order" problem for you.
How to mix two PDFs
- Add File A: the odd pages (the fronts).
- Add File B: the even pages (the backs).
- Keep Reverse file B ticked if the backs were scanned from the last sheet to the first, which is what happens when you flip the whole stack over. Untick it if both files are already in the right order.
- Check the preview line, which lists the new order, such as "A1, B3, A2, B2, A3, B1".
- Click Mix & download.
Why the backs are usually reversed
Imagine a stack of three sheets. You scan the fronts from sheet 1 to sheet 3. Then you flip the whole stack over like turning a page of a book: the pile now has sheet 3 on top, and its back faces up. Scan again and the first back you get is page 3's back, then 2, then 1. So file B is in the reverse order of file A. Ticking "Reverse file B" corrects that automatically, so you get front 1, back 1, front 2, back 2, and so on. If your scanner works differently, untick the box and check the preview.
When page counts differ
If one file has more pages than the other, the tool warns you and puts the extra pages at the end. That usually means a blank back was skipped by the scanner, or a page was scanned twice. In that case, look at the preview order: you may want to fix one of the files first, for example by removing a duplicate with Delete PDF Pages, or by adding a missing blank. If the counts differ because the last page has no back, the leftover front at the end is correct.
Other uses
- Inserting a slip sheet or separator between every page of a document.
- Combining a document with its translation so each page is followed by its other-language version.
- Interleaving originals and annotated pages for review.
- Collating odd and even pages that were printed or exported separately.
A worked example
You've scanned a 20-page agreement: fronts.pdf has 10 pages and backs.pdf has 10 pages. Add them as A and B, leave "Reverse file B" ticked, and look at the preview: A1, B10, A2, B9, A3, B8, and so on. Mixed this way, the output is the complete 20-page agreement in the right order. Open it and spot-check three places: pages 1 and 2, somewhere in the middle, and the last page. If they read in order, you're done. If not, untick the reverse option and try again.
After mixing
With the pages in order, tidy the file. Rotate any sheets that came out sideways, crop away the scanner edges, remove blank backs that carry nothing, then add page numbers. A final compression makes the file easy to email. If you simply need to place one whole file after another, Merge PDF is the right tool, and if the whole document is backwards, Reverse PDF flips it.
Privacy and limits
Both files are read and combined in your browser; nothing is uploaded, which matters for scanned contracts and personal paperwork. Password-protected files must be unlocked in your reader first. The pages are copied exactly as they are, so quality doesn't change, and rotated pages keep their rotation.
Troubleshooting
If the result looks scrambled, the usual cause is the reverse option being wrong for how you scanned: flip the checkbox and look at the preview again. If some pages are missing, check whether the scanner skipped blank backs, and fix the files before mixing. If the pages are sideways, rotate them after mixing. If you scanned the stack by flipping each sheet individually instead of the whole stack, both files are in the same order, so untick the reverse option.
Scanning tips for next time
If your scanner has a duplex option, use it; it avoids all this. If not, scan the fronts, then flip the stack as a whole and scan the backs, and keep a note of which is which. Name the files "fronts" and "backs" straight away, and count the pages in both so you know they match. A minute of care at the scanner saves much more at the computer.
If the result still isn't right, test with a tiny sample first: scan two sheets, front and back, mix them, and check the order. Once you've confirmed which reverse setting your scanner needs, you can process big stacks with confidence and note the settings for next time.