The HPE files are exports of a Favorites list. As K4EET noted, these are binary coded files, and would look like a hot mess if you tried to open one with a text editor program.
You can use Notepad or Notepad++ to open an HPD file, but it likely will not be in a format that is pleasing to the eye.
Excel is a little better on HPDs:
Software such as ProScan can print HPD files in a format that would be better. I don't have ARC536, but I believe you can also choose to print with it as well.