XtraReports reporting tool natively supports export to PDF, so it is likely that you can print your .prnx reports as .pdf documents using the program's default PDF export function which is the most convenient way to achieve prnx to pdf conversion.
A viable alternative would be to use some viewer of .prnx files (if it exists) that also has at least printing capability and use a virtual PDF printer to export prnx to pdf. Unfortunately, we do not know about any at this time.
Updated: June 13, 2023