Hi Karen,
Yes, as you noted your visitors would need to have Digital Editions installed. They'd also need to make a local copy of your newsletters and then add them to the Library. (From the Library view there's an "Add Item to Library..." option in the menu.)
Having said that, I'd really like to provide you with an approach like you'd see on the demo library, where there's a badge that says, "Read" where it opens the Newsletter in Digital Editions, I just don't know how to do that (yet.)