Prevent apps from changing the default app to open PDF files?
Try rebuilding your Launch Services database.

In Terminal:

/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user


Then reboot immediately.

References: 


https://www.tekrevue.com/tip/rebuild-launchservices-fix-duplicate-entries-os-xs-open-menu/
https://eclecticlight.co/2017/08/11/launch-services-database-problems-correcting-and-rebuilding/