I'm not using PMPDF, and the distributed files seem to be mixing versions of both important files, but it may be a missing feature of PMPDF to be able set an own name. To override the name generated by DBE and/or PMPDF.
AFAICT, according to "HELP WPSREF WPPRINTER" and without using PMPDF, JOBDIALOGBEFOREPRINT or OUTPUTTOFILE can only offer a dialog at best. Not the best solution when the number of invoices exceeds one or two.
FWIW: in a Windows environment, with several involved different "solutions", I'd probably expect a human interface, opening and checking each PDF file and saving it with a better file name. So even post-processing PDF files would be quite "innovative", if that's a normal standard.
Regarding PMPDF, is it clear which files are the right version(s)? Files sizes are different too. IIRC one file in the WPI archive is newer, and one file isn't.