Interleave two PDFs - rebuild a duplex document from two scans
First file (front sides)
Second file (back sides)
Drop the fronts in the first slot and the backs in the second, and leave "Reverse it" on. A stack flipped as a block feeds back last-page-first, so the backs are usually in reverse order.
When you scanned the backs in the same direction as the fronts - for example by turning each sheet individually, or with a scanner that already un-reverses them.
That is fine and common - the last sheet often has a blank back that the scanner dropped. The pages interleave as far as they pair up and the surplus is appended in order.
Page content and geometry are preserved exactly. Bookmarks and form fields from either file are not carried into the mixed document.
