Adding pages to preview PDFs

How are you adding the blank page? Are you using the Edit ➙ Insert ➙ Blank Page menu option?

If you are and it's not working boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

You can only drag and drop a blank PDF document, or a compatible image into Preview when adding pages to an opened PDF. This saves and reopens with the newly added content as a PDF.