Originally Posted by user_none
There exactly the same as what's sent to the Kindle and they work on the Kindle. I will look into why they don't work for apps.
I was just playing around and got the apnx files to work in the kindle for PC app, the .apnx file needs to be named with the ASIN of the book. example "B002953482_EBOK.apnx" which is the ASIN number I gave "A Modern Utopia - H. G. Wells.mobi.