OK. Same issue here I think... Recently I noticed all PDF file icons were defaulting to to Affinity Photo. I used "Get info" on one PDF file and used the "Open with" to reassign to Preview by default, and then hit "Change all". PDF file icons started showing up as Preview again, but trying to open any PDF resulted in Preview getting focus, then a few seconds of nothing, followed by Finder animating the file icon as if it was opening, then nothing... not even an error. Restarted. Nothing. Did some disk first aid, nothing. Hitting space for a quick preview of a PDF worked fine, reassigning PDF's to another app worked fine, reassigning back to Preview and nothing. Opening other image formats in Preview worked fine (jpg, png), opening PDF's in other apps worked fine. I did some of the suggestions listed here, deleting Preview preferences and also the Preview Container directory, logged out and back in again and still no improvement. I created a new test user and logged in – PDFs opened fine in Preview there! Hmmm... It felt like a securities/permission type bug so logged back into my own account (PDF's still failed to open in Preview) and followed https://support.apple.com/en-gb/HT203538 Initially after following the steps and logging out and back in, still no luck. I gave up at this point, and tried to get on with some work. About an hour later I had to restart as a couple of apps were now unexpectedly starting giving the pinwheel of death opening files. After the restart, PDF's were opening again in Preview! I'm going to assume the guidance to "Reset permissions" procedure in Finder followed by a:
diskutil resetUserPermissions / `id -u`
...may have done the trick, and just required a full restart and not just a logout and login again, however at some point in desperation I did also try a:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
...so can't be 100% as to what the fix may have been. Good luck!