So, if we want to make the Kindle file readable on any device, remove the DRM from the KFX file first.

The KFX file is not compatible with the Apple Book app, you can read the DRM-free EPUB book and the purchased book from the iTunes Store on the Apple Book app.

KFX is one of the Kindle ebook formats, which is the successor of AZW3, which enhances content layout presentation and DRM.

All the ebook files on Kindle are DRM (Digital Right Management) protected, so they can’t be opened on other unauthorized devices.

These confusions push me to search some information online and finally I am clear about them: Moreover, Apple Book couldn’t open the ebook files I uploaded. Because, I am not sure what KFX file it is, whether I should transfer it to iPad or not. However, it is much more complex than I thought. Rather than buy a new Kindle, I decided to import those ebook files into my iPad mini, since the reading experience is pretty good on Apple Book. I backed up all the files in the Kindle, when I found out that it easily runs out of power, which indicates its battery will be dead soon.

Kfx drm