'There was an error opening this document. The root object is missing or invalid.'

[#1]

Hello! I am trying to open a PDF file with the Adobe Acrobat Reader DC and it says 'There was an error opening this document. The root object is missing or invalid.' Could someone please help me? That document has a lot of important notes (I am a student) and I really need it ASAP 😞 

** I am a Mac user by the way, using Catalina right now. The Adobe Acrobat Reader DC version I'm using is Version 2020.013.20064

[#2]

This sounds like a corrupted or improperly created PDF file. How was it created (software, operating system, etc.)?