Safari forces you to manually "trust" new websites. This means that when you first attempt to view PDFs from a website or domain you've never visited before, Safari versions 6.1 and 7 will not automatically allow the PDF plugin to load. When this error occurs, the browser window is gray or blank and it displays the alert: "Blocked Plug-in."
You must enable the Acrobat and Reader PDF viewer plugins by trusting the plugin globally or for each website.
Safari 6.1 and 7 is included with Mac OS X Mavericks 10.9. There is a plug-in manager that allows you to enable and disable plug-ins globally or per site.
Here is a step-by-step guide from Adobe on how to enable this feature.