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Plaintext is readable as-is, without any explicit conversion step. That being said, if you really want a code review device, you might want to look at a cheap laptop or tablet, for one basic reason: there's no direct text editing available on the Kindle, and annotations are painful to write because of the keyboard. The DX doesn't even have numbers on the main keymap, so you have to use a symbol shift to type them.

Thanks, but I read code mostly on paper today. My idea was to replace that.

That's something I hadn't considered, but if it's good at it, it could swing me into spending the money... seconded!

annotations do not work with pdf's at the moment.

and AFAIK, the font choices available outside of pdf make showing code less attractive.