Add-On Dev: Open external file with default program I was planning to create some simple help-like text or HTML documents in a local folder of my add-on, and provide "help" buttons on the panels to allow the user to access them from each section that relates to that file.

Is there a way to open these files using the user's default program from within Blender?
You can add a wiki URL ("wiki_url") to the bl_info of your add-on. This will result in a link to the documentation in the add-on preferences. Alternatively you can use bpy.ops.wm.url_open(url="") in your code to open the browser.