Start by not swallowing your exception: when you use a blank catch, and do nothing useful with it, you throw away all the information you need to identify the problem and solve it.
Do this:
try
{
...
}
catch (Exception ex)
{
... log the exception, display it in a message box,
... or stuff it on your web page - we have no idea
... what environment you are working in
axAcroPDF1.src = null;
}
Then, when the code is run, you can look at the exception and hopefully see what is going wrong. But without that, you are working in the dark...