The HP Paintjet line of printers uses a type of bitmap graphics, apparently in conjunction with the PCL printer command language, which documents some features involving transmission of bitmaps.
Apparently such bitmaps have been known to be stored in files with a .pjxl extension, but not much is documented online about them; trying to google it mostly yields the rash of SEO-friendly script-generated sites that put up pages on every conceivable file format written in Mad-Lib style: "Are you trying to open a (PJXL) file? You may want to know what a (PJXL) file is. Well, Joe's Grand File Format Documentation Site is the foremost expert in documenting (PJXL) files. A (PJXL) file is a(n) (HP PAINTJET PCL BITMAP GRAPHIC). How do you open a(n) (HP PAINTJET PCL BITMAP GRAPHIC)? Try these file conversion programs: (NONE LISTED). Or your best approach is to click on the file and hope your operating system is configured by default to open something useful. (Praying a lot might help.)" Perhaps it goes on to advertise a commercial file recovery service that specializes in (PJXL) files... honest!
All things considered, this site is admittedly not being all that much more helpful to you in figuring out what to do with those oddball PJXL files you may be struggling with. But it does link to the PCL article, which in turn links to some documentation manuals which might possibly help you. If you find some solid information, it would be nice if you'd register an account on this wiki and participate here by writing up what you've found!