In Finder > Preferences > Sidebar, under DEVICES, check the top box which will be your assigned computer name. This will make it available to you in an open Finder window.
Open a new Finder window, scroll the left panel down until you see the DEVICES section, and single-click on the name of your computer. The contents of the main Finder window will change and you will see a network globe, and for our focus, a silver icon of a disk drive with your boot drive name — probably Macintosh HD.
Double-click Macintosh HD. Double-click Library. Scroll down until you see Internet Plug-Ins, and double-click that folder. Holding your shift-key down, select the following icons:
Now, with both icons selected, right-click to bring up the contextual menu and choose Copy 2 Items. Then, click once on your desktop, press and hold the option key with right-click. This time, you will see Move 2 Items Here. Select this and the plug-ins will be moved to your Desktop.
You now have a choice of default Safari PDF viewing, or if you prefer the Adobe PDF plug-ins, reinstall Adobe Reader 11.0.04 (as of this post date) to reinstate the Acrobat PDF browser plug-ins. Once this is working, drag the Adobe plug-ins on your Desktop to the trash.