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

abracadabra-DocID has changed its name (DocID) 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 the PDF file.
abracadabra-DocID is free.

• The first string, before the comma, 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, after the comma, 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.