Some people do not like Safari's new ability to open PDF files in the browser window.
Here is the default to change in the Terminal to switch back this behavior:
defaults write com.apple.Safari WebKitOmitPDFSupport -bool YES
[
robg adds: To use this hint, first quit Safari, open the Terminal, and type the above command. Restart Safari, and you should find that PDFs are treated as they were before. To restore Safari's handling of PDFs, repeat the above command, changing
YES to
NO. Remember to quit Safari first. Note that I haven't tested this one; I use the Acrobat PDF plug-in and haven't had a chance to disable it in order to test...]