Can no longer open some pdfs

I installed Big Sur on my MacBook Pro a few weeks ago. Generally no problems - I work from home as an editor, easily open PDFs in Preview (which I know is not an Adobe product). Just tried to open a family-history document that I spent hours and hours scanning a few years ago - it says "The file "Jim & Carol Donnelly history book.pdf" couldn't be opened because there is no such file." 

Why would all the other PDFs I work with be openable, but not this one? I also logged into my icloud.com account - still no luck opening it from there. I am ready to cry or lose my mind, thinking of what all may have been vaporized when I upgraded to Big Sur. (Which I had to do, in order to sync with a new printer.) 

Thanks for any help anyone can provide.