This tools allows you to merge 2 pdfs produced from scanning the fronts and backs of a document. The first pdf has the fronts and the 2nd pdf has the backs. This tool assumes the backs are in reverse order as they would be if you flipped the stack over on the scanner, but it allows you to reverse the order of the pages if necessary.
If you like this tool and would like to support me making more like it, consider clicking my Amazon affiliate link and buying whatever you would normally buy.
Note: This is all done locally, so the pdfs never leave your browser (they are not transmitted to any server).
Note: This probably won't work on a mobile browser (iphone or android) because of the combination of the file size of scanned documents and the memory limitations of the mobile browser. I could make a version that works on mobile, but I would probably have to charge you for it.