Your scanner has a document feeder but it only scans one side of the paper. So you fed in the stack, got a PDF of all the front sides, turned the stack over, fed it in again and got a second PDF of all the back sides. Now you need one file where page 2 is actually the back of page 1.
Merging the two files won't do it, because a merge puts one file after the other. What you want is interleaving: front 1, back 1, front 2, back 2. That's what Alternate & mix does, and it runs in your browser, so the scans aren't uploaded anywhere.
Why the backs usually come out backwards
This trips up nearly everyone the first time.
Picture five sheets. The first pass scans the fronts in order: 1, 3, 5, 7, 9 (counting in final page numbers). The sheets land in the output tray, you pick up the stack and flip it over to feed the backs. Flipping the whole stack also flips its order. The sheet that was at the bottom is now on top, so the feeder grabs the back of the last sheet first. Your second PDF reads 10, 8, 6, 4, 2.
If you interleave that as it is, you get 1, 10, 3, 8, 5, 6 and so on, which is nonsense. The fix is to read the backs file from the end: take its last page first. In the tool that's the "Reverse order" checkbox on the file.
Not every scanner and tray works this way, so don't assume. The live preview shows you the result before you commit, and checking the first four pages there takes ten seconds.
Interleaving two scans step by step
- Open Alternate & mix.
- Drop the fronts file into the box labelled "PDF A" ("Drop the first PDF here").
- Drop the backs file into "PDF B". You can also drag both in at once and sort out which is which afterwards.
- On the PDF B card, tick "Reverse order" if your backs came out last to first.
- Look at the preview. Each thumbnail has a coloured badge like A1 or B5 that tells you which file and page it came from.
- Click "Mix PDFs" and download the result.
The file is saved as the name of PDF A with "-mixed" added, so fronts.pdf becomes fronts-mixed.pdf.
A quick sanity check before you download: in the preview, page 2 should carry the badge of the last page of B if you reversed it (B5 in the five-sheet example), and page 4 should be the one before that (B4). If you see B1 in position 2 and the content doesn't match, flip the "Reverse order" setting and look again. There are undo and redo buttons too, so nothing you try is permanent.
If the files are in the wrong slots, use the up and down arrows on a card to swap them. The order of the cards decides whose page comes first.
When the two files have different page counts
The tool doesn't stop when one file runs out. It keeps taking pages from whichever files still have some, so any leftover pages from the longer file end up together at the end.
That's helpful for a mismatched pair of scans, but it also hides mistakes. If your fronts file has 12 pages and your backs file has 11, something went wrong during scanning: usually two sheets fed together, or a sheet jammed and you rescanned it. The result will look fine at the start and drift out of step from the point of the error. Compare the page counts on the two cards (each one shows "12 pages" or similar) before you mix. If they differ, find the problem sheet first.
Another common cause is the scanner's own "skip blank pages" setting. If some sheets have blank backs and the scanner drops them, the backs file is shorter and everything after the first blank is misaligned. For this job, turn blank page skipping off in your scanner software, interleave, and then remove the blank pages afterwards with Delete pages.
Fronts and backs in one file
Sometimes both passes end up in the same PDF: the scanner kept appending, so pages 1-10 are fronts and 11-20 are backs.
You can handle that without splitting first. Add the same file twice. On the first card, type 1-10 in the "Pages" box. On the second, type 11-20 and tick "Reverse order". Each card only uses the pages you list, and the preview shows the interleaved result as usual.
The "Pages" box takes the same syntax as other EddiesPDF page tools: commas between parts, ranges like 3-7, 8- for "page 8 to the end", and "last" for the final page. A descending range like 20-11 also works, which does the same thing as ticking "Reverse order".
Mixing in other patterns with Take
Each card also has a "Take" number, which is how many pages are taken from that file before moving to the next. It starts at 1, which gives you the strict A, B, A, B pattern.
Changing it covers a few less common jobs:
- A two-page form per person in file A and a one-page photo ID per person in file B. Set A to Take 2 and B to Take 1, and each person's ID follows their form.
- Inserting a separator sheet. Put the separator in file B, type 1, 1, 1, 1 in its "Pages" box (one per section), and set Take on file A to the size of each section. The same page can be listed more than once.
You aren't limited to two files either. "Add another PDF" adds a C, D and so on, and the tool goes round them in card order. Three-way mixes get confusing fast, so lean on the preview.
What this tool doesn't fix
Alternate & mix moves whole pages. It doesn't rotate, straighten or clean anything up. If the backs came out upside down (common when the stack is turned end over end instead of side to side), open the mixed file in Organize pages and turn the back pages the right way up, or see the guide on rotating PDF pages. If the scans are slightly crooked, that's what Deskew PDF is for.
It also isn't the right tool for joining documents end to end. For that, use Merge PDF; the merging guide covers ordering and bookmarks.
The mistake to check for last
Open the mixed PDF and look at the last two pages, not the first two. Errors in interleaving start small and get worse, so the start of the document can look perfect while the end is out of step. If the final back side sits right after its own front, every page in between is almost certainly right too.
Also useful: Delete pages · Organize pages
