How to recombine front and back scans into one PDF

Two passes naturally produce two files

A duplex scanner captures both faces while each sheet travels through the feeder. A simplex scanner captures only the exposed face, so a stack needs a separate pass for each side. Each pass creates its own file; the document is not complete until those files are combined.

Why the backs may appear in reverse order

Paper paths and output trays do not all stack sheets the same way. After the front pass, flipping the entire stack can make the back of the final sheet enter first. The second PDF then runs from the last back to the first. Do not assume this always happens: inspect the first few thumbnails and use the order shown by your scanner.

Interleave the pages in reading order

The finished PDF should alternate front 1, back 1, front 2, back 2 and so forth. If File B starts with the final back, reverse it before interleaving. Thumbnail previews are valuable here because they reveal both an incorrect reversal and a stack fed in the wrong orientation.

Reordering does not require rasterization

Interleaving copies whole PDF pages into a new sequence; it does not need to inspect their pixels. Text, vectors and images can therefore remain in their existing form. Even so, exported PDFs should be checked for links, forms, annotations and digital signatures, since restructuring a document can affect interactive features or invalidate signatures.

Checks before you download

Different page counts often signal a skipped or double-fed sheet. Compare both files with the paper original rather than relying on the tool to append unmatched pages correctly.

Interleaving changes order, not orientation. Rotate any upside-down backs as a separate step.

Either file can require reversal because scanner designs and flipping methods vary. The thumbnail sequence is more reliable than a universal rule.

Two-pass scanning is a practical way to digitize both sides with a simplex device. Once you establish the order produced by your own scanner, recombining future stacks becomes a predictable process.