I love E Ink devices for reading ebooks (ePub / Kindle), but they aren't the best solution for PDFs, in my view. First, of course, you're dealing with a reduced-size display, even with the Kobo Aura One; second, many PDFs have color illustrations or text, which will be seen only as grayscale; third, the screen refresh isn't spritely enough on E Ink devices.
The best PDF reader out there, although it costs a lot, is the 12.9-inch iPad Pro, which will show you a US letter-sized or European A4 page FULL SIZE (not reduced), in color, and with lightning fast page turns and zooming in and out.
http://www.apple.com/ca/ipad-pro/