Not all ebook readers can open all ebook file formats. What you can do when your ebook reader cannot open MOBI, AZW, LRF or FB2 files is to convert it to a widely supported, common, and popular format.
Converting your documents to EPUB can be helpful as well. That way, you can read articles, reserach papers, and more on the go. Just convert from PDF to EPUB. Want to publish your writing or share it with fellow authors and avid readers? Convert your Word document to EPUB. Of course, ODT to EPUB is also possible.