How can I recover missing PDF files?

I was trying to transfer all my pdf pattern to usb drive but book app didn’t let me . The book app page kept getting frozen. I want to transfer all my pdf to my usb drive to make space for app Books. Then I went to check today they all gone just 15 pdf left the rest are all gone. Is away I can get them back to transfer all my pdf to my usb drive. I start to check since Feb. 17, 2025 to transfer them but didn’t let me every time I was trying.