If you really need something for PDF you might be better off with a 10" tablet. That's not nearly as good for reading books but it's a lot better for PDF. Also it'll cost a lot more than $60 but if you can manage it it might be worth it.
I bought a Lenovo full HD 10" tablet from Amazon for $160 8 or 9 months ago. They still have it at the same price. This would be as good as you can get for PDF with it's high pixel density and 10" screen. It's been a really nice tablet. The only problem has been a somewhat flakey Wifi but it's only a problem occasionally. Otherwise it's been great. Here's a link to it:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Barry
Barry