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Alternate & Mix two PDFs

Pick the front and back scan files, interleave in one click, everything stays on your device.

How Alternate & Mix two PDFs works

PDF Alternate Mix interleaves the pages of two PDF files in alternating order, A1, B1, A2, B2, …, so you get a single merged document that pairs each page from the first file with its counterpart from the second. This is exactly the operation you need after scanning double-sided documents on a single-sided scanner: one pass gives you the odd pages, a second (reversed) pass gives the even pages, and this tool stitches both stacks back together in the right sequence with pdf-lib.

A reverse option flips the second file before interleaving, which matches how duplex back faces come out. If the two PDFs differ in length, it interleaves up to the shorter one and appends the remaining pages, so nothing is lost. To split a PDF instead, or to reorder pages freely, see the PDF split and PDF page organiser tools.

Frequently asked questions

What is this tool actually for?

Its primary use case is reassembling a double-sided scan: after scanning the front faces of all sheets and then the back faces (often in reverse), you have two PDF stacks. Alternate Mix zips them together in the correct front/back order so every sheet reads naturally.

What if my two PDFs have different page counts?

The tool interleaves up to the length of the shorter file and then appends the remaining pages of the longer one at the end, so you never lose pages from either document.

Can I reverse the order of the second PDF before mixing?

Yes. A reverse option lets you flip the second file's page order before interleaving, which is the standard need for duplex scanning, where the back faces come out in reverse stack order.

Are my files uploaded to a server?

No. Every operation runs inside your browser tab using JavaScript and WebAssembly. Your file is read into memory on your own device, processed there, and the result is offered as a local download. Nothing is transmitted to Sunasty or any third party. You can verify it yourself: open your browser DevTools, go to the Network tab, and watch, no request carries your file.

Does it work offline?

Yes, once the page has loaded. Because the processing happens on your device, you can disconnect from the network and the tool keeps working. This is the clearest proof that your data never leaves your machine.

Is it free? Do I need an account?

It is completely free and requires no sign-up. There are no watermarks, no daily limits and no tracking cookies needed to use the tool.