The iPad suggestions are good for things that work out of the box and without fuss, but here's a cheaper alternative. I use an old thinkpad yoga with linux. Linux uses a lot less memory and processor than windows/iOS so it works fine on old cheap hardware. (But very old very cheap hardware might have trouble with modern websites.) A convertible laptop also has advantages over an ipad, with a built-in keyboard, a ton of USB ports, an HDMI port, etc, so you can also use it as your main laptop.
I use xournal++ for taking notes on pdfs and it's great.
The big downside with linux is that installing it takes some work (maybe just an hour if you're lucky), and sometimes things don't work out of the box. But with enough time and effort any problem can be fixed, and you can customize just about anything to your liking.
