How to rebuild a double-sided scan from two PDFs
A front-side scan and a back-side scan cannot be joined with a normal merge. The second stack often needs to be reversed, then both files must be interleaved one page at a time.
Open Alternate & MixForward A. Reverse B. Take one from each.
- Add the front or odd-page scan first.
- Keep the first PDF Forward and set the second PDF to Reverse.
- Use one page per turn, then inspect the first, middle, and final pairs.
Why the backside scan is often reversed
Imagine a three-sheet document numbered 1 through 6. A single-sided feeder captures the front sides as PDF A: 1, 3, 5.
After you flip the stack, the sheet that was last in the first pass may enter the feeder first. The back pass can therefore arrive as PDF B: 6, 4, 2.
A regular merge gives 1, 3, 5, 6, 4, 2. Alternating without reversing gives 1, 6, 3, 4, 5, 2. Reverse B to 2, 4, 6 first, then alternating produces the intended document.
1 → 2 → 3 → 4 → 5 → 6A reliable scan-to-PDF workflow
Keep the source scans until the exported document has passed the final check.
- Scan every front or odd-numbered side into PDF A.
- Flip the same stack and scan every back or even-numbered side into PDF B.
- Open Alternate & Mix and add PDF A first, then PDF B.
- Keep PDF A set to Forward.
- For the common flipped-stack workflow, set PDF B to Reverse.
- Keep Pages per turn at 1 for both files and run the mix.
- Download the result and inspect it before deleting either source scan.
When PDF B should stay forward
Do not reverse a file only because it was added second. The thumbnails decide the setting.
Keep B Forward
Use Forward when the first thumbnails already read 2, 4, 6. Some feeders preserve sheet order, and some scanner software reverses the pass automatically.
Set B to Reverse
Use Reverse when the last thumbnail belongs immediately after page 1. In the example above, B reads 6, 4, 2 and must be reversed.
PDF A can also be reversed if the front pass came out last-to-first. Direction follows the visible page order, not the file name.
What if the page counts do not match?
An odd-length document often leaves PDF A with one extra front page. VaultPDF alternates while selected pages remain in both sources, then keeps unmatched selected pages at the end instead of dropping them.
Pages per turn can be changed per source for unusual patterns, such as two pages from one file followed by one from another. Page ranges can also exclude covers, separators, or test scans.
Watch for omitted blank backsides
If a scanner silently skips a blank back side in the middle, every later pair can shift. Mixing software cannot infer where that missing page belonged. Add a blank placeholder first, or reorganize the affected pages afterward.
Verify the exported PDF
Treat the first download as a proof, not the final archive.
- Check the first four pages for the expected front-and-back sequence.
- Check a pair near the middle, where an omitted blank becomes obvious.
- Check the final pages for extra covers or unmatched pages.
- Confirm rotation and orientation.
- Compare the output page count with the total selected source pages.
Alternate mixing changes order; it does not replace checking that every physical sheet was captured.
The document stays in your browser
VaultPDF processes the selected PDF content on your device. The Alternate & Mix workflow does not upload the document to a conversion server, and no account is required.
This guide documents the behavior of VaultPDF's own Alternate & Mix tool.Ready to rebuild the scan?
Add the front scan first, inspect the back scan's direction, and keep both originals until the mixed PDF is verified.