abracadabra-DocID is a plugin for Acrobat (Reader, Pro & Standard)

has changed its name (IDENTIFIER) and is now part of the free abracadabraTools.
Download it on this page: https://www.abracadabrapdf.net/?p=111

abracadabra-DocID adds a quick comparison tool that displays the “Permanent identifier” [ID] of PDF files.

abracadabra-DocID is free.

• The first string is a permanent identifier based on the contents of the file at the time it was originally created; it does not change when the file is incrementally updated.
• The second string is a changing identifier based on the file’s contents at the time it was last updated.
• Comparing the Permanent identifier [ID] of several PDF files quickly corroborates: exactly the same, not the same at all, or a variant.