AxAcroPDF and OnMessage / postMessage

Question

I have a pdf opened in an AxAcroPDF control and am trying to capture a posted message from JS in the pdf. Here is my javascript code....

try
{
   app.alert(this.hostContainer);
   var x = new Array();
   x[0] = "Hello";
   this.hostContainer.postMessage(x);
}
catch (err)
{
   app.alert(err.message)
}

The first message that pops up says "undefined". The second message (through error catching) says "this.hostContainer has no properties"

What am I missing? Do I have to somehow assign the AxAcroPDF control to be the hostContainer? Or maybe hostContainer is only for parent html pages?