Hi ArminCh,
Thanks for reaching out in Apple Support Communities. I understand that while viewing a PDF in Preview you're unable to use the dictionary as a reference. I know this is an important tool to be able to use; I'd like to help. If you're referring to being unable to use the "Look up" feature when you highlight a word, I'd recommend using these steps to start your Mac in safe mode and test for the behavior: Use safe mode to isolate issues with your Mac
If the issue seems to be resolved in safe mode, restart your Mac normally and test for the behavior again. If the issue persists in safe mode or after restarting, test for the behavior in a new user account: How to test an issue in another user account on your Mac
Let me know what you find.
Best Regards.