The document reader or the physical e-reader? For the latter, PineNote is still in development stage. For document reading, you will have to use Calibre + some plugins to convert kfx files, which still can’t be read by readers. You can also download the books directly from the Kindle store in other formats, but I don’t know how that would do.

https://www.dedoimedo.com/computers/calibre-kfx-drm-guide.html