Books is suddenly opening all my PDF's. How to stop?

Books is finding all of the PDFs on my iMac. I just want books to show the PDF files I add to books, not EVERYTHING. Because of this it is nearly impossible to manage my collections and select files to synchronize with my phone and iPad. I have looked and tried solutions already posted but none of them work. Changing which app opens pdf files does nothing (Preview is used for opening all my pdf files already). This is a very cumbersome problem for me. Hopefully someone has a real fix.