This appears to be an issue where an older version of pdf is no longer compatible with the newest Acrobat Pro and, when the user tries to edit the pdf, this error comes up.

https://community.adobe.com/t5/acrobat/adobe-acrobat-dc-has-encountered-an-unknown-error/td-p/8995354


All pdfs created by Unit4 Business World and Student World Milestone 7.0 (a.k.a. Agresso) have this issue.

Work-around step-by-step guide

  1. Open the pdf in Acrobat Pro
  2. Save as... an MS Word document (.docx)
  3. Open the Word document in MS Word (edit it here or not; user's choice)
  4. Save as... a PDF document (.pdf)
  5. Open in Acrobat Pro and now it is editable, which the user may not care about if they've already edited it in Word.