On the first attempt to view a PDF on a new domain or website in Safari 6.1 and 7 (delivered with Mac OS X Mavericks 10.9), the browser window is gray or blank. It displays the alert: "Blocked Plug-in."
The latest versions of Safari use a new plug-in manager for enabling and disabling plug-ins on a global or per site basis. The Acrobat and Reader PDF viewer plug-ins are not trusted by default until you actively trust the plug-in globally, or for each website.
For each domain or website on which the "Blocked Plug-in" message appears: