Thanks to many posts on focal point about the topic I was able to come up with a pdf drilldown url that seems to work with multiple pdf viewers (adobe, foxit, chrome and firefox defaults, etc.). It also works across servers. This is the url:

Reporting Server:
-SET &URL = &URL_PROTOCOL | '://' | &SERVER_NAME | &FOCEXURL

MRE:
-SET &URL = &URL_PROTOCOL | '://' | &SERVER_NAME | &FOCEXURL | '&|IBIF_ex=[pathtoyourfex]&|CLICKED_ON=&';
 

I can copy the fex between environments and it works just fine. You may also need &SERVER_PORT if you are not using the default ports 80 or 443.

In the administration console custom settings I had to set &SERVER_NAME and &URL_PROTOCOL to pass so that I could use those variables:

 
<SET> &SERVER_NAME (pass)
<SET> &URL_PROTOCOL (pass)
 

Then in the fex's style sheet I set the appropriate item to have a url drilldown:

 
URL='&URL'( \
...
),
 

Let me know if this doesn't work for any popular PDF viewers that you might use OR if this doesn't work/isn't necessary in future releases.