I do. I have a library of PDFs ever at hand that way. Over the years I've picked up a few Fires, and I have them in 7, 8 and 10-inch.
The new 2020 HD8+ has a lovely display, nice and clear, able to display even an A4 page clearly enough to read comfortably without having to zoom in. The extra RAM is welcome.
HD10 was already good enough for this purpose. It's bigger and heavier of course, though still not as much as a hardback book.
I find the 7s a bit small, but serviceable in a pinch, and certainly handy, although the displays are a bit glare-prone.
Install Play Store if you can and get Xodo for the best PDF handling in my opinion. If you want to keep it as-is, there are a few decent if uninspired PDF readers available in the Zon store. Foxit might be a good choice in that case, even though it's a bit pricey; at least it's stable, capable and ad-free.