pdf to pdp conversion is probably related to export of documents from PDF to Postscript format, typically for the reason to print the document in different physical printer device.
For graphics files it's best to use graphics software, Adobe Photoshop or similar tools from Adobe comes to mind. But if it's just text documents, you can probably do that many PostScript drivers. Of course for Photoshop's specific .pdp files, only the original software can help you out.
Updated: September 22, 2023