I am having the same issues. I think it all stems from Safari 5.1, which doesn't support Adobe. Here's the site discussing it: http://kb2.adobe.com/cps/908/cpsid_90885.html#products
It's damn frustrating!
MIght be helpful to disable Adobe plugin by going to Adobe Reader's Preferences -> Internet and unchecking "Display PDF in browser using".
Then dump the atrocious bloated Reader.
See this https://discussions.apple.com/thread/3252667?start=0&tstart=0 for further solutions.