Back up all data.
In the Finder, select Go ▹ Go to Folder... from the menu bar, or press the key combinationshift-command-G. Copy the line of text below into the box that opens, and pressreturn:
From the folder that opens, remove any items that have the letters “PDF” in the name. You may be prompted for your login password. Then quit and relaunch Safari.
If you still have the issue, repeat the above steps using this line:
If you don’t like the results of this procedure, restore the items from the backup you made before you started. Relaunch Safari again.
So you didn't find any files like: Adobe PDF Browser plugin (or just PDF Browser) plugin in the /Library/Internet Plug-Ins folder ??
If that's the case, right or control click a PDF file you have stored on the hard drive then click Get Info.
Click the pop up menu next to: Open with
If it's not available from the menu click Other the navigate to the Applications folder then select Safari.
Click Change All
edited by: cs