I think I have this one figured out. From virtualized web browsers, I have never been able to use the Adobe Reader X plugin. The Virtual app would start to load any pdf and then crash with a message about a corrupt sandbox (ZAV application sandbox). This would happen if Adobe reader was installed on the machine or virtualized as a shared component. This may be because ARX runs out of its own sandbox. I have now got this working, I will do some testing from some other Browsers.

Application = Virtualized IE8 Configured from a recipe using ZAV 9

Fix = add the following to the "Child Process exception list"
Acropdf.dll; Acroiehelper.dll; Gbdetect.dll; and Pdfshell.dll

Virtual IE8 can now display PDF's using the web plugin. I will test with firefox and report back.