This is a somewhat recent problem that has started to crop up. We hope to identify the cause of the problem and have it fixed soon.
In the mean time, a work around is to right click on the PDF Link and Select Save link As to save the PDF to your computer, then open the saved file.
To help us in tracking down the problem, can you please post back a few more details about you system:
What web browser and version are you using?
what version Of Acrobat Reader do you have installed?
Do you have Acrobat Pro installed?