A lot of office scanners have a sheet feeder but no duplex scanning. Feed a stack of double-sided pages through and you get the fronts. Flip the stack, feed it again, and you get the backs. Now you have two PDFs, one holding pages 1, 3, 5 and the other holding 6, 4, 2, and what you wanted was a single document that reads 1, 2, 3, 4, 5, 6.
Merging the two files end to end does not help, because that gives you every front followed by every back. What you need is to interleave them: take one page from each file in turn. This guide explains how interleaving works, why the back scans usually come out reversed, what happens when the two files have different page counts, and how to do the whole job with the Alternate Mix Pages tool. It sits in the PDF tools collection alongside the merge, reverse and blank-page tools mentioned below.
TL;DR: To combine separate front and back scans, interleave the two PDFs: page 1 from the fronts file, then page 1 from the backs file, then page 2 from each, and so on. Because flipping the stack reverses the backs, turn on "Reverse PDF 2 order" so the backs run 2, 4, 6 instead of 6, 4, 2. The PDF page interleaver shows the resulting page order before you download, appends leftover pages when the files have different lengths, copies pages without re-encoding, and runs client-side. It is free, no signup.
What does it mean to interleave two PDFs?
Interleaving two PDFs means building one document that alternates pages from each source: first page of A, first page of B, second page of A, second page of B, and so on until both run out. It is sometimes called an alternate merge, a collate merge or mixing pages.
A normal merge concatenates. With a 3-page file A and a 3-page file B, a merge gives A1, A2, A3, B1, B2, B3. An interleave gives A1, B1, A2, B2, A3, B3. Same six pages, different order.
The Alternate Mix Pages tool always starts with the first file. Whatever you load as PDF 1 (A) takes positions 1, 3, 5 and so on, and PDF 2 (B) takes positions 2, 4, 6. For a duplex scan, that means the fronts go in PDF 1 and the backs in PDF 2.
Why do back-side scans come out in reverse order?
Because of how paper stacks. When you scan the fronts, the feeder takes sheet 1 first and the output tray stacks the sheets so that the top of the output is the last sheet. To scan the backs you pick up that stack and turn it over, and now sheet 3's back is on top.
Take a three-sheet original. Sheet 1 has pages 1 and 2, sheet 2 has pages 3 and 4, sheet 3 has pages 5 and 6.
| Pass | What the feeder sees, in order | Resulting file |
|---|---|---|
| Fronts, stack face up | Sheet 1 front, sheet 2 front, sheet 3 front | Pages 1, 3, 5 |
| Backs, stack flipped | Sheet 3 back, sheet 2 back, sheet 1 back | Pages 6, 4, 2 |
Some feeders and some flipping methods produce the backs in the right order, so do not assume. Open the backs file and look at its first page. If it shows the back of the last sheet, you need the reverse option.
What page order do you get, with and without reversing?
Here is the same three-sheet example run through the tool both ways. A is the fronts file (1, 3, 5) and B is the backs file (6, 4, 2).
| Setting | Tile preview | Actual page content in order | Correct? |
|---|---|---|---|
| Reverse off | A1, B1, A2, B2, A3, B3 | 1, 6, 3, 4, 5, 2 | No |
| Reverse on | A1, B3, A2, B2, A3, B1 | 1, 2, 3, 4, 5, 6 | Yes |
With the reverse option on, PDF 2 is flipped before interleaving, so its last page pairs with PDF 1's first page. The preview labels show this directly: B3 appears in position 2.
Notice the tell-tale sign of forgetting to reverse: the middle page of the document is in the right place (page 4 in position 4 here) and everything else in the backs is mismatched. If you open a result and only the centre looks right, turn the option on and download again.
How to combine front and back scans, step by step
- Scan the fronts into one PDF and the backs into another. Keep the stack in the same order between passes; do not shuffle or remove sheets.
- Upload the fronts in the PDF 1 (A) box, by clicking or dragging the file in. The box shows the file name, size and page count.
- Upload the backs in the PDF 2 (B) box.
- Check the options. Once both files load, an Options section and a Resulting Page Order Preview appear. Turn on "Reverse PDF 2 order" if the backs were scanned from a flipped stack.
- Read the preview. The tiles show the first 20 positions, colored by source, with "+N more" for longer documents and a total page count underneath.
- Click Download Interleaved PDF. The file saves as
interleaved.pdf.
Open the result and spot-check three places: the first two pages, a pair in the middle, and the last two pages. If all three pairs are fronts followed by their own backs, the whole document is right.
What if the two PDFs have different page counts?
The leftover pages from the longer file are added at the end once the shorter file runs out. Nothing is discarded, and the tool does not fail.
This is common with duplex originals that end on a blank back, because many scanners skip blank pages automatically. Suppose a four-sheet original has seven printed pages and a blank back on the last sheet:
- Fronts file (A): 1, 3, 5, 7
- Backs file (B), flipped stack, blank skipped: 6, 4, 2
With reverse on, B becomes 2, 4, 6, and the output is A1, B1, A2, B2, A3, B3, A4, which is pages 1, 2, 3, 4, 5, 6, 7. The leftover A4 lands at the end, where it belongs.
The same rule causes trouble when a blank back is skipped in the middle of a document. If sheet 2 had a blank back that the scanner dropped, the backs file has one page fewer and every back after that point is paired with the wrong front. The preview cannot detect this, because it only knows page counts, not content. There are two fixes: turn off blank-page skipping in the scanner settings before the backs pass, or add a blank page to the PDF at the missing position in the backs file before interleaving.
Other uses for alternating pages
Scan repair is the main job, but interleaving fits any pair of documents that should alternate.
| Use | PDF 1 (A) | PDF 2 (B) | Reverse? |
|---|---|---|---|
| Duplex scan from a simplex feeder | Fronts | Backs | Usually on |
| Bilingual document | Pages in language one | Matching pages in language two | Off |
| Questions and answers | Question sheets | Answer sheets | Off |
| Slides with notes pages | Slides | Notes, one per slide | Off |
| Before-and-after comparisons | Original versions | Revised versions | Off |
| Interleaved lined paper for notes | Printed handout | A PDF of lined pages | Off |
For the last case, the second file needs at least as many pages as the first, or the final handout pages will have no note page after them.
What happens to quality, bookmarks and links?
Pages from both documents are copied without re-encoding. Scanned images keep their resolution and compression, text stays selectable, and page size and orientation stay as they were. A letter-sized front and an A4 back can sit next to each other, because page size is a property of each page rather than the document.
The output is a new PDF, though, so document-level structures do not come across.
| Element | In the interleaved PDF |
|---|---|
| Page content, images, fonts | Unchanged |
| Page size and rotation | Kept per page |
| External web links | Still work, they belong to their page |
| Internal links between pages | Break, because page positions change |
| Bookmarks from either file | Dropped |
| Fillable form fields | Lose interactivity |
| Title, author and other metadata | Empty |
The PDF page model behind this behavior is defined in ISO 32000-2: each page is its own object with its own resources, and the page tree simply lists them in order. The tool, built on the pdf-lib library, copies page objects into a fresh page tree, which is why page content survives and document-level structures like the outline do not.
Is it private, and does it work offline?
Yes on both counts. Both PDFs are read from your disk into browser memory, the new document is assembled in memory, and the result is handed back as a download. Nothing is uploaded to a server, and after the page loads the tool keeps working without a network connection.
Scans of signed contracts, ID documents, medical forms and bank statements are exactly what people feed through a sheet scanner, so this matters. The originals on your disk are not modified.
The practical limit is memory. Both source files and the output exist in the tab at the same time, and scanned PDFs are large, so a 300-page scan of each side may be slow on a phone or low-memory laptop. On a desktop browser it is rarely a problem.
Troubleshooting common problems
A file does not appear after upload. The file could not be read as a PDF. The usual causes are a password-protected (encrypted) file, which is refused at load time, or a damaged download. Remove the password in the software that set it and upload again.
Every other page is wrong but the middle is fine. The backs are in reverse order. Turn on "Reverse PDF 2 order".
All the backs are off by one after a certain point. A blank back was skipped mid-document. Insert a blank page at that position in the backs file, or rescan with blank skipping off.
Back pages are upside down. Some flipping methods rotate the sheet 180 degrees. Interleaving does not change rotation, so rotate the PDF pages in the backs file first, or rotate the affected pages in the combined file afterwards. The guide to rotating PDF pages covers choosing which pages to turn and by how much.
The fronts and backs were swapped. If the result starts with a back, you loaded the backs as PDF 1. There is no swap button, so reset with the circular arrow in the header and upload them the other way round.
Interleave, merge or reverse: which tool do you need?
Three PDF operations get mixed up because they all change page order.
| You have | You want | Use |
|---|---|---|
| Two files that should alternate page by page | One file, A1 B1 A2 B2 | Alternate Mix Pages |
| Several files that should follow each other | One file, all of A then all of B | Merge PDF |
| One file in backwards order | The same file, last page first | Reverse PDF Pages |
| One file with a few pages out of place | Specific pages moved | Organize PDF |
Sometimes two steps are simpler than one. If the backs file is the only thing in the wrong order, you can reverse the page order of that file on its own and then interleave without the reverse option. The result is the same. And when a document arrives as several scan batches, merge the PDF files per side first, so you end up with one fronts file and one backs file, then interleave. The guide on how to merge PDFs in the right order covers that first step. For small fixes after interleaving, such as moving a stray page, the organize PDF guide walks through reordering and deleting pages.
When the two scans are ready, interleave the front and back pages, check the tile preview against the order you expect, and spot-check the first, middle and last pairs before filing the result.
