Back up all data before making any changes. Please take each of the following steps until the problem is resolved.
If Adobe Reader or Acrobat is installed, and the problem is just that you can't print PDF's displayed in Safari, you may be able to print by moving the cursor to the the bottom edge of the page, somewhere near the middle. A black toolbar may appear under the cursor. Click the printer icon.
There should be a setting in its preferences of the Adobe application such as Display PDF in Browser. I don't use those applications myself, so I can't be more precise. Deselect that setting, if it's selected.
If you get a message such as ""Adobe Reader blocked for this website," then from the Safari menu bar, select
Safari ▹ Preferences... ▹ Security
and check the box marked
Manage Website Settings...
and make any required changes to the security settings for the Adobe PDF plugin.
Triple-click anywhere in the line of text below on this page to select it, the copy the selected text to the Clipboard by pressing the key combination command-C:
In the Finder, select
Go ▹ Go to Folder
from the menu bar, or press the key combination shift-command-G. Paste into the text box that opens (command-V), then press return.
From the folder that opens, move to the Trash any items that have "Adobe" or “PDF” in the name. You may be prompted for your login password. Then quit and relaunch Safari.
The "Silverlight" web plugin distributed by Microsoft can interfere with PDF display in Safari, so you may need to remove it, if it's present. The same goes for a plugin called "iGetter," and perhaps others — I don't have a complete list. Don't remove Silverlight if you use the "Netflix" video-streaming service.
Do as in Step 4 with 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.