I know that, if you want to watch movies or videos, there is a program called VLC media player that lets you read many many media file types. I am looking for something like this for document files (pdf, epub, ebook, doc, docx, rtf, html, txt, ... )
Thanks in advance if you know such a thing
Calibre supports all of these formats.
I don't really know what you mean by the 'ebook' format, but Calibre can read EPUB, MOBI and AZW formats.