I haven't seen it said yet, so try this:
"Set Default Printer" to Microsoft Print to PDF first.
Then "Print Document"
This will bypass you even having to open the file, it should take you straight to a Save As PDF screen.
I apologize, I completely misread the question. Try to focus the "Save Print Output As" Window. After that, if you are still not in the "filename" text box, try using Send Keys {Tab} to get to it. If that still isn't working, you could always use Move Mouse to Image. For this screen, I grab the "File Name:" and some of the whitespace to the right, then tell the click to go far right center in the Advanced Options.
Best of luck!