From your Safari menu bar click Safari / Preferences then select the Extensions tab. If you have any installed, turn that off, then relaunch Safari. Try a PDF file.
If it's not Extensions, open a Finder window. Select MacintoshHD in the Sidebar on the left. Now open the Library folder then the Internet Plug-Ins folder. If you see any files such as this:
AdobePDF Browser or Viewer Plug-in
Move it to the Trash, relaunch Safari.