When scanning a double-sided document in a non-duplex scanner, you end up with two separate PDFs - one for the odd-numbered pages and one for the even-numbered pages. Combining them in Acrobat Pro creates a single document with the pages in the order "1-3-5-7-....2-4-6-8-...", meaning that we have to manually move all of the pages into the correct order, which is not feasible for hundred-page long documents. We need a 'combine' function that automatically interleaves the pages so that they end up in the order "1-2-3-4-5-6-7-8-..".