Me neither Cheryl. I did both things as you suggested. Still no joy. [By the way, the problem is with the Mac Mini].
If I click on a link to a PDF file a new tab appears with the correct heading for the PDF file but the page is just black.
To answer MisterMe, I have no problems opening PDF files in Safari on the MacBook Pro.
In fact I have no problem opening up PDF files using Firefox on the Mac Mini either. However, I like Safari and tend to use that 95% of the time so it's bit of a pain that I can't see the PDF files using it.