A few relevant screenshots might help...
How do you open screen Show_PDF?
Would it probabky be possible to stay on the screen to show the pdf? Look into using a virtual screen, see tip 1 General Tips and Tricks for App Inventor
The url address you are looking for, is it the same information whjch is passed to screen Show_PDF, or is it something else?
Provide an example of those...
Taifun