[madars]

You can use Calibre to convert your ebooks to MOBI/AZW3, which Kindle accepts, and if you put your Kindle in airplane mode (so it stops getting OTA updates) and wait log enough it'll probably get a jailbreak at some point. At which point you can install KOReader for superior PDF/DjVu reading experience. Last jailbreak covers Kindle software <= 5.14.2; so Kindle Voyage and earlier all should be supported. (See https://www.mobileread.com/forums/showthread.php?t=346037 HN discussion: https://news.ycombinator.com/item?id=31220553)

[systemicdanna]

This looks very promising. In addition to using KOReader I will be able to customize the lock screen and remove Amazon's upsells. I guess the only downside is that I won't be able to use WiFi or buy e-books from Amazon.

[Vrondi]

Also, Koreader runs on many brands of ereaders, so it may be worth exploring if you're interested. Kobo readers can read non-DRM epubs out of the box and can also run Koreader. Check out the MobileRead forums. Here's the one for Kindles: https://www.mobileread.com/forums/forumdisplay.php?f=140