Pretty much any graphics file or document can be printed to PDF format and .wwd files (X Windows bitmaps) are certainly no exception in this. The only thing that is needed for such xwd to pdf conversion is to find graphics tool that can open your .xwd files and use PDF printer to product the document.
You can find numerous tools advertised for this all over the Internet, but what you truly need is just a graphics viewer like XnView and simply use the print function and virtual PDF printer.
Updated: August 14, 2023