Your browser is simply doing as instructed. When directed towards a file, it must either display the contents of the file or download it. In this case, it is displaying the contents of the file. Here are a few things that you could do to improve the situation:
1. Urge your visitors to buy a Mac and use Safari (it styles and reads RSS feeds for you).
2. Urge your visitors to use Firefox and install the “Sage” extension: https://sage.mozdev.org/
3. Edit any template that has the RSS link and add feed:
in front of the RSS link. For example: <a href="feed:<?php bloginfo('rss2_url'); ?>">RSS</a>
feed:
is a new protocol which, theoretically, should launch the user’s default RSS reader. This will not work on all systems.
4. Add a javascript popup to the feed link which explains the link and how it must be used in an RSS reader and offer “Ok” to continue and “Cancel” to (of course) cancel. Please don’t ask me how to do this. I’ll leave that to the experts. For example, see the RSS link at: https://txfx.net/
5. Add some CSS styling to your feed to make it look more like a regular web page: https://fernando.dubtribe.com/archives/2005/05/14/rss-style-for-your-wp-feed/
or
6. Sign up with Feedburner and use their “Browser Friendly” feature: https://www.feedburner.com/ An example of a “Browser Friendly” feed: https://feeds.feedburner.com/macmerc