O link a seguir diz que deve ser bastante simples ... Basta mover ou excluir um arquivo ...
First, close Safari and launch Finder. Press Command-Shift-G (or choose Go > Go to Folder from Finder’s menu bar) to open the Go to Folder window. Enter the following location and press Return:
/Library/Internet Plug-ins/
OS X Finder Internet Plugins Library
The contents of this folder will vary depending on the number and type of your Safari plug-ins, but look for the files called AdobePDFViewer.plugin and AdobePDFViewerNPAPI.plugin (note that depending on your version of Acrobat, you may only have the first file).
OS X Safari Acrobat PDF Plugins
You now have a choice: if you hate the Acrobat Safari plug-in and want to return to the OS X default PDF viewer forever, simply delete both files above. If, however, you’d like to preserve the option to go back to the Acrobat Safari plug-in in the future, move both files out of the Internet Plug-ins folder and back them up in a new folder elsewhere (your user Documents folder, for example).