To change the file type that OS X outputs when using the command + shift + 3 or command + shift + 4 (with or without the spacebar after) in Tiger launch Terminal, and depending on what file type you want outputted, type the appropriate line below followed by return:
defaults write com.apple.screencapture type pdf
defaults write com.apple.screencapture type png
defaults write com.apple.screencapture type jpg
defaults write com.apple.screencapture type tif
Quit Terminal. One caveat: You must restart your computer for the change to take effect right away. To revert to the default png format, type:
defaults write com.apple.screencapture type png
as shown above, or delete the com.apple.screencapture plist file in your user preferences folder (again, you need to restart). You can test out different formats, just remember to restart your computer when you are done or after each test.