Say you fill out an on-line PDF file, but the data cannot be saved with the form. What do you do? print it on paper. What if some users don't like hard copy. You print to a file, which is a PRN file for most printers. But the PRN file formats vary from printer to printer. HP's prn file is not PS, while most other printer manufacturers do use PS format. So you need 2 kinds of viewers to open PRN files.
XPS printer is a virtual printer which intercepts data to form an image file. This image file can be viewed with a web browser. Isn't that universal access?