I would start by defining a new file extension for documents created with my add-in. When Windows encounters an unknown file type, it asks the user to identify an application to use for opening it, or offers to search the Web for a suitable application. Your download site will have to publish its ability to provide a suitable program for opening that type of file, and I have no idea how that is done, but I'm sure you can find that information using Google.
Will Rogers never met me.