pdf files won't open

Back up all data.

Triple-click the line of text below to select it, the copy the selected text to the Clipboard (command-C):

/Library/Internet Plug-ins

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, remove any items that have the letters “PDF” in the name. You may be prompted for your login password. Then quit and relaunch Safari, and test.

The "Silverlight" web plugin distributed by Microsoft can also interfere with PDF display in Safari, so you may need to remove it as well, if it's present.

If you still have the issue, repeat with this line:

~/Library/Internet Plug-ins

If you don’t like the results of this procedure, restore the items from the backup you made before you started. Relaunch Safari again.

You will eventually lose any data that isn't backed up, whether you take my suggestion or not. If you're willing to reinstall all your applications from scratch when that happens, you don't need to back them up.

There is no model that's uniquely best for everyone. Every design is a compromise. I suggest you go to an Apple Store and try them out.