If you purchased a physical disc, the serial number is usually on a sticker on the back of the CD sleeve or inside the box.
If your old PC still has PaperPort 12 installed but you can’t see the full serial, a free tool like Magical Jelly Bean KeyFinder can reveal it from the registry without you manually navigating. Run the tool, look for “PaperPort 12,” and copy the key before decommissioning that PC. paperport 12 serial number