Oboku use https://prose-reader.com/ which is a flexible engine made to support any kind of contents. There is no hard limitations on what can be supported and if your book does not work let us know.
Currently known supported formats
.zip archives (comics, audiobooks, etc)
.cbz archives (this is a zip archive)
.rar archives (same support as zip)
.epub files (they are zip archives, commonly used for regular books)
