Sometimes you have to install Adobe Reader because some pesky government agency forces you to use a pesky PDF file that only works with Adobe Reader. And once you install Adobe Reader, it will take over your whole Mac, even the default PDF preview in Safari.
Here’s how to restore the default PDF preview on OS X:
/Library/Internet Plug-ins (there is a “Go To Folder…” option in the “Go” menu of the Finder if you need it).
In that folder, delete the following 2 plugins:
Restart Safari and you’re done ;)