Eink solves the eye strain issue, since it uses real ink and is either unlit or front-lit.
Amazon still sells the Kindle DX for $200, which is probably fine for most PDF's, though it may choke on some of the bigger ones or you may have to orient it sideways due to the low resolution of the DX screen.
The new Kindle Voyage is more than high definition enough to handle pretty much any PDF, but you'll probably have to zoom or read sideways unless you like looking at tiny text, since unlike the DX, the screen is small, but it is also modern, up to date, and much higher resolution.
The Ectaco Jetbook color 2 is much pricier (around $500), but has a larger screen like the DX and is high resolution with color support. I haven't used it, but it seems to be a good PDF reader, although a bit pricey.
Sony has released a very nice, super high resolution, black and white eink reader. It is the most ideal one I have seen for reading PDFs (due to its large size and ultra high-res screen) but it will cost over $1000 to get it imported.
I use my old Kindle 2 for PDF's since every Kindle can read them, but unfortunately, until they come out with a successor to the Kindle DX (something like an extralarge paper-white or Voyage), the market is kind of limited unless you want to spend more than $200 on an ereader, in which case the Sony and Ectaco readers look pretty good.