I use Sumatra frequently, and while it's great at what it does, there's a lot it doesn't do. It's strictly a viewer only, so it doesn't even have rudimentary annotations like highlights or underlines. As an epub/mobi reader it's even more lacking. It won't allow you to search for or copy words, despite being able to do so with pdfs. Also, there are some azw3 that are effectively mobi, but Sumatra won't load them unless you rename them with the mobi extension.