On your computer, the license file is typically stored in C:\ProgramData\ABBYY\FineReader\12\Licenses . Note that this folder might be hidden.
Often found on "key shop" sites for suspiciously low prices, these are often valid Corporate keys that were sold to a business and then leaked or resold illegally. While they may initially activate the software, they are time-bombs. Once ABBYY detects that a single corporate key is being used by hundreds of disparate IP addresses, they will blacklist the key in their activation server, rendering the software useless overnight. Abbyy Finereader 12 Professional Serial Key