.e-pub (electronic publication) is a widely used format and has been proposed as an international standard. The very best reader for it it is Calibre. It can read and convert all formats (including word, PDF and others). It is however only available for Windows, Mac and Linux. For Android devices you have to download a reader like ReadEra (my favorite).

Kindle has also switched to the e-pub standard and it available for all platforms.

These apps are free to download and use. for help with any of them please refer to their websites.