There seems to be a solution, reinstalling Mountain Lion, but I wouldn't try it myself, since I'm a rookie in Macs (only been using them for a year or so). Maybe, if you're more familiar with Macs...
Unfortunately, all the solutions that Mr. Davis kindly suggested, didn't work
Funny thing is, I must log in my Mac as a guest and Safari works properly (being a guest in my own "home", lol).
Go to the Library folder in your normal (non-guest) account. It's hidden, but if you hold down the option key while clicking on the Go menu in the Finder, you'll see it.
Go to the Preferences folder.
Remove these four files, or whichever ones are there.
Launch Safari, and view a pdf on Dropbox. Mine is now normal.