In 2015 Amazon introduced an enhanced typsetting format (format 10 or KFX) to be the successor of KF8. It produces files with an extension kfx. The enhancements include such things as soft hyphenation, improved character placement and word spacing, a more adaptable page layout, and approximate page numbering. It also includes the new Bookerly font. Amazon has not as yet offered a tool like kindlegen that will produce this format. Kindle Previewer 3 can read this format, but stores its results in a different format KDP that is similar to the KDX format. EPUB documents can be converted to the KFX format using the program Calibre with an appropriate plugin. The plugin uses Kindle previewer 3 to produce output in the KDP format that is then converted to the KDX format.
To convert epub files to kfx files it is necessary to have both Calibre and Kindle Previewer 3 installed on your computer. Kindle Previewer 3 can be downloaded from Kindle Previewer and Calibre can be downloaded from Calibre. To install the kfx plugin open Calibre and go to Preferences > Advanced > Plugins. Click on the button “Get new plugins”. Select “KFX Output” and click “OK”. The plugin will be installed.
To convert your EPUB document to KFX load your EPUB document into Calibre by clicking on “Add Books”, entering the path to your document, and clicking on “open”. Next click on “Convert books”. In the Output format dropdown menu select KFX. On the left are a number of options. The KFX Output plugin creator suggests that you select “Tablet” under Page setup > Output profile in order to prevent images from being scaled to small. I also check “Create approximate page numbers” under KFX Output. To start the conversion click “OK”. In the status bar jobs will go to 0 when the conversion is complete. In Windows the Output is stored in the folder c:\users\<user-name>\Calibre Library.