My original posting about Preview app not opening PDFs appears to have gone into the ether.
However, my problem is that my PDFs (Genealogical records) amount to some many thousands of pages (eg. 170 MBs to over 156,000 pages of 374MBs) and Preview just will not open them (it goes into not responding requiring a force quit command).
If I go to Adobe Acrobat Reader DC they will open almost immediately!!
These files previously worked well with Preview in the older systems but since Mojave & Catalina I have had to resort to using Adobe Reader - much preferred the older method and look of Preview.
Much smaller PDFs that I may have, work OK with Preview (ie, jpg, png.)
I have tried all the tricks recommended above like re-setting permissions etc even spending some time with Apple techs on the phone including screen sharing but..............
Very frustrating