Please share your system specification, what is the operating system you are working on and exact version of the Acrobat Application.
You can check that by Launch Acrobat>Help>About Acrobat and check for the update as well(if any available)
Also, the document that you are trying to compare are they pdf files or in a different format?
Greetings! I am sorry to hear that you are facing issues with the compare feature.
It would be great if you could help us with a few queries that will help us in investigating the bug:
• What is the version of Acrobat Pro that you are using?
• Can you please share the file which you’re selecting as both ‘Old File’ and ‘New File’ so that we can try to reproduce the issue at our end?
• Can you please confirm that this issue is happening with that specific file or with other files also?
• Also, please can you confirm that issue is happening on some other machine, if you’ve one with you?
Adobe Acrobat Team.
Getting version of application (Acrobat )
1. Click on 'Help' menu in application.
2. Click on 'About Adobe Acrobat…'