I've found the Kindle Fire to be a pleasant reading device for PDFs and ebooks. It's not an e-ink display, but you do get full color and PDF rendering is good. It's also cheap compared to an iPad, especially if this is your primary use case (I have both an iPad Pro and a Kindle Fire, the Kindle is my main reading device now, the iPad is a portable computer with Scrivener, Blink, Textastic, and Working Copy).
A nice thing about the Fire is that it's cheap. They had a recent sale going on the Fire HD for $60, I think it's about $100 now. It's not an either-or proposition (at least not for me), more of an "iPad now, Kindle in a month or two" (or reverse depending on your more important use-cases) proposition.
Ecstatic. I've had 3 previous kindle iterations, but the Oasis is the first one I outright prefer to paper books (except for aesthetic or nostalgic purposes). Splashproof, fast page-turns (incl physical buttons which I usually prefer to swiping), endless battery life, comfy form factor, x-ray, whispersync.... thought I was splurging when I bought it at release but it turned out to be a bargain. Go for it! :)
I find Apple Books to be the only way I can read ebooks consistently. It seems dumb but I think the ability to freely fiddle with the "page" without turning it is what does it for me. I've got an e-reader but I kind of hate reading on it. Paper or Apple Books, specifically, for me.
