Secure pdf and Safari problem

I am having a problem where a secured PDF can't be viewed/open in Safari. The screen greys out but no pop up appears to enter password, as expected.

It works fine with all browsers: Chrome, Firefox, Opera, even buggy IE from version 6 up. But Safari 5.1.3 doresn't?! It's usually the other way around...

The PDF was created using Adobe Acrobate Pro, but works fine on Preview.

Any solution from the community would be truly appreciated! Thank you.