I'm reporting an issue I had, but I also have the fix for others. I had it where all of my downloads (be it a PDF, zip, vcard, etc.) would display the download's contents as if you opened it in a text editor in the browser itself.
The fix was related to WP Super Cache. I needed to take the download endpoint I set in DM's settings & add it to WP Super Cache's list of rejected strings. So "/downloads" was added in my case, and now it downloads & displays the files correctly.
Not sure if there's something that could be done to the download handler to prevent this conflict. It'd be nice if there was.