Hey there dalago,
Thanks for reaching out to Apple Support Communities. I understand you’re having issues opening PDF documents after updating to macOS Catalina. I’ll be happy to help.
Check the permissions of the file that you can’t open and make sure you have permission to Read & Write. This link has more info:
If you don’t have permission to open a file or folder on Mac
If that doesn’t help, see if this occurs in safe boot or a new user account to isolate and possibly resolve this issue:
Use safe mode to isolate issues with your Mac
How to test an issue in another user account on your Mac
Also, where did you get this PDF from? If you download a PDF from a different site, can you open it? You can download this PDF from Apple’s website and see if it work for you:
iPod touch User Guide for iOS 8.4 - PDF
Let me know if any of these steps help.