Newbie Mac tip: On Mac OS X, Adobe Acrobat Reader installs itself as a Safari plugin, overriding Preview as the in-window PDF viewer. If you subsequently try to uninstall Acrobat Reader, PDFs in Safari go broken. To go back to Preview for viewing PDFs in Safari, quit Safari, then trash /Library/Internet Plug-ins/AdobePDFViewer.plugin. (Thanks Apple support forums.)