Hello, I have used this plugin to convert KPF files to KFX files. The KPF files were created using Kindle Create from random PDF files I have. I then used this in an AppleScript to automate sending PDFs to my Kindle Scribe so that I can write on them directly. This is very hacky but it works for now until there is a smoother way of getting PDFs onto the scribe so they can be written on, without contacting Amazon (this works fully offline on my mac, the only requirements are that the Kindle Create app is installed, and calibre is installed with the KFX Output plugin.)
If a Kindle is connected the resulting KFX file is automatically copied to it.
Link to the GitHub repo with relevant files:
https://github.com/willemml/kindle_scribe_pdfsender
Again, this is very hacky and only works on macOS, but I thought someone might find it useful anyways.