Adobe has decided that when you install Adobe Reader in OSX that it will use it's own software, Adobe Reader, as the default PDF viewer in Safari. If you prefer to use Preview in Safari instead of Adobe Reader then perform the following commands below.
cd /Library/Internet\ Plug-Ins/
sudo rm -r -f AdobePDFViewer.plugin
sudo rm -r -f AdobePDFViewerNPAPI.plugin/