You don't need to root the HDX to read ePubs, or to adjust base font size in the Kindle app. Just learn to use calibre to liberate and convert your AZW3 files to ePub, or use it to change the base font size in mobis/kf8s. That doesn't require any rooting, nor does installing Mantano Reader, or any other reader app, on the HDX. You might have to use another Android device to get the app file, but it will install after you side load it to the HDX. Mantano is my favorite independent Android reader app, but there is also FBReader, CoolReader, and others available that are pretty good.