There is a send to kindle app [0] which has worked for every document I have ever tried and it works almost instantly. I use it a ton. It even works for epub books I have converted to mobi through calibre (doesn't work perfectly all the time but if you got some programming/hacking skills, you can make it work. I got a book that mobi kept trying to translate each paragraph into a separate page through a triple conversion from epub to single page html doc (then vim find/replace page breaks) and then to mobi).

[0]: https://www.amazon.com/gp/sendtokindle