I'm a (new to) Mac user running Safari as well.  My experience is slightly different when trying to open a saved PDF file that's attached to, for instance, an expense:

• Open expense record
• Click on PDF attachment
• Opens new browser window and just spins and spins... 
• There is no option to download the attachment (when I try, it gives me an HTML link...)

Is anyone having a similar experience?  This is super frustrating, and the only way I can open PDF attachments currently is to whip out my old Windows laptop...