I have had both a Kobo and Pocketbook. You can convert anything to .epub with Calibre, and websites you can easily convert and send articles to read with Readeck automatically.
You can also use something like FrogFind for the built in browser on the ereader. use https://send.djazz.se to easily transfer ebooks to your reader, also has options to turn it into a kepub (optimized kobo epubs) if that is your device