Quit Safari, launch Terminal, and type the following:
defaults write com.apple.Safari WebKitOmitPDFSupport -bool YES
Press return and then relaunch Safari. It will now download PDF documents to your hard drive. You can revert to Safari 2.0s default PDF behavior by entering this command with NO instead of YES .
or
Download OnyX. It's a free 4.1MB utility, and one of its MANY MANY features is to allow you to not have Safari automatically open .pdf files.
Install it, open its 'Appearance' tab, click 'Safari', and there you have it. Remember, this utility is free. It's a must-have for any Mac OS X user, in my opinion.
or
Remove the Adobe acrobat Viewer plugin located in
/Library/Internet Plug-Ins/AdobePDFViewer.plugin