[bityard]

You may find that "budget" and "acceptable for PDFs" don't often ride the same train.

Most small e-readers are _much_ slower to render PDFs at all, to start with. On a small e-ink screen, you'll find the speed of the page refresh gets annoying as you try to pan around the document. Larger-format readers (10"+) tend to be on-par with tablets in terms of price, and offer fewer features beyond better battery life.

That said, for regular reading, I'm fond enough of the Kobo Clara 2E that I bought one for everyone in the household last year. If you're handy (hacky?), you can use them without creating an account with Kobo. And you can put any non-DRM book you like on it just by connecting it to a computer and dragging and dropping it over. (No need to mess with Calibre.)

My own personal experience with reading and eyestrain suggest that the media itself may not be your issue. If it was a contrast or brightness issue, there tools built into the iPad to fit the display to your comfort level. Consider a visit to an optometrist. My wife is far-sighted and has no problems reading computer screens and anything else a couple feet away, but got terrible eyestrain while reading books until she got glasses.