This app was mentioned in 3 comments, with an average of 2.67 upvotes
I do plan to add support for multiple files in my little Reader app but I'm super busy at the moment. I still wonder why there are no developers creating something like that.
Bah, none of the obvious readers appear on the (new) Play store on the watch, from which an iphone user could install directly.
You may have to get an APK and sideload it onto the watch. Maybe Reader?