Preview stopped working as default app for opening PDF files
When I clicked on a PDF file in Finder (or email attachment), it would open with Preview.
This used to work for decades but not any longer: PDF files now open with some other PDF reader such as Acrobat reader, Skim... instead of Preview (although PDF files can still be opened from inside Preview).
I have tried to repair as follows: select PDF file, open file information (cmd-I) Open with... Preview and check box Change all.
It works but the problem will eventually reappear (maybe after I restart or relog, I do not know for sure).
Setting System preferences > Default apps > Extensions ... pdf ... Preview does no better.
I also encountered the likewise problem of JPEG files not opening with Preview but with some other JPEG reader.
How to permanently reset Preview as the default application for opening PDF and JPEG files?
MacBook Pro, OS X El Capitan (10.11.6), null