When you create your xfaForm variable you need to configure a render service (under Advanced Settings).
The render service is responsible to render the actual form (xdp) into a PDF. Usually you choose the renderPDFForm from the Process Management to render the pdf.
If you look at that renderPDFForm process, you'll see that it uses the variable /process_data/@formUrl to specify the url to the xdp. This variable is set when you configure formURL property of the render service (under Advanced Settings).
Usually we set it to Task->Form URL, but you could use your own variable there that contains the path to your XDP.
Jasmin