I have an iPad Air that I don't use much. (After I got it I started using my iPhone as I had envisioned using the iPad--frustration of smaller screen beats lugging an extra device around for my purposes). But to the point, in what little I have used my iPad, it is as smooth and pleasant experience as my iMac and my iPhone.
I have had horrible experience with PCs and I (rightly or wrongly) equate all tablets and smartphones other than Apple products with that. I have a friend who has the newsest (I think a new one may be about to come out though) Samsung Note (huge phone--a phablet). It freezes up and has issues akin to what drove me away from PCs.
How big of a screen do you want? I will be upgrading my old 4S to an iPhone 6 Plus pretty soon. I think that screen will be big enough to even more comfortably do what I had envisioned for my iPad and lug one device around instead of two. Don't overlook the possibility that a 6 Plus may accomplish what you need and double as your phone.
I read online material, documents, PDFs, books, etc. on my little 4S screen all the time. I am sure the 6 Plus is going to be very nice for such use for me.
As for my iPad, it is a great device. I intend to start using it more as laptop substitute.