No, the pdf files are local on my Mac.
"Do these files have non-ascii characters in their names like é, ü, or some Asian or other script characters?"
it does not make any difference...
What's strange is that it's completely random: if I request that all pdf's be opened with Preview, sometimes it works, sometimes not.
If I ask for them to open with SKIM, it never works!
I should mention that I was on Catalina before and I have just migrated to Monterey.
On Catalina, I have never had any problem opening pdf's on SKIM or Preview.
So, elementary the logic says it is a system problem! Obviously Apple has again changed something in the system that causes this bug!
I can't understand why they don't test and fix such a simple thing that handicaps a myriad of users!
Thank's for your help.