How to use the Interleave PDF
- Add your front-side and back-side scan files into the processing queue.
- Use the queue arrows to ensure that your front pages sit in position 1 and back pages sit in position 2.
- Keep “Second file is in reverse order” enabled if your scanner inverted the back pages, or untick it if they are already in forward order.
- Press Interleave PDF to combine the pages and download your completed interleaved.pdf file.
Merging two-sided office paperwork from single-sided scanners
Scanning multi-page double-sided contracts, invoices, or employee records on a basic office feeder usually requires two separate passes: first scanning all front sides (pages 1, 3, 5, etc.) into one PDF, and then scanning all reverse sides (pages 2, 4, 6, etc.) into a second file. Merging them with standard tools simply places the entire back stack after the front stack, leaving the document completely out of order.
Interleaving solves this administrative headache by alternating pages between documents: drawing the first page from the front scan, then the first page from the back scan, then the second from each file, continuing until all sheets are assembled. This creates a cohesive double-sided digital record ready for filing, client distribution, or audit review without demanding manual page reorganisation.
Automatic reversal of back-page scanner stacks
When staff take a stack of scanned front pages, flip the physical pile over, and feed it back into the scanner, standard automatic feeders draw sheets from the top, causing the reverse sides to be scanned in backwards order. For a 6-page contract, the second scan file typically contains page 6, followed by page 4, and finally page 2.
To compensate for this physical scanning reality, the option “Second file is in reverse order” is switched on by default. The engine reads the second file backwards from its last page to its first, perfectly aligning page 1 with page 2, page 3 with page 4, and page 5 with page 6. If your scanner outputs back sides in regular ascending order, simply uncheck this option. When interleaving three or more files, the reversal rule applies to all files after the first.
Queue management, uneven page lengths, and file safety
The alternating order strictly reflects the file order in your queue, which you can easily modify using the up and down arrows. A minimum of two PDF files is required. If one document contains more pages than another—such as a contract with an odd number of sheets where the final page has no backside—the remaining pages from the longer document are appended to the end of the output.
All pages are assembled losslessly with pdf-lib without recompressing images, modifying vector artwork, or altering searchable text layers. The finished document downloads as interleaved.pdf. Password-protected files must be processed with Unlock PDF first, and completed files can be passed forward to Remove Blank Pages from PDF or Compress PDF using Continue with.
Questions
How is interleaving different from standard PDF merging?
Standard merging places entire documents one after another. Interleaving alternates pages turn by turn, drawing the opening sheet from the first document, then the first sheet from the second document, proceeding in continuous rotation.
Why does the tool assume my second scan file is in reverse order?
Flipping a stack of paper to scan the back sides in a standard sheet feeder naturally inverts the page order. The default setting corrects this backwards sequence automatically.
What occurs if my front scan has an extra page compared to the back scan?
The tool alternates pages until the shorter document is exhausted, and then attaches the remaining surplus pages from the longer file to the end of the output.
Are sensitive business records sent to a cloud server during interleaving?
No. The entire process runs locally inside your browser using pdf-lib. Your documents remain strictly on your workstation and are never transmitted over the network.