My PDFs are locked

I have a Mac laptop and have been creating and using PDFs for many years. I normally use Preview to open and read PDFs without any problems. All of a sudden my PDFs are locked and require a password to open. I have tried every password I've ever used on my computer but none of them work. I would like to emphasize that these are MY files, not someone else's, so no one else locked them. I downloaded Adobe Acrobat Reader but it won't open the files either. It's very frustrating.

Thanks!