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).
Thank you. I still might hold on for an updated iPad Mini because I know I'll get the itch to set up Blink 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.
I want a device I just use for reading, but I almost exclusively read PDFs of technical books. I'm thinking iPad Air vs Kindle