No one has figured out a way to objectively measure eyestrain so it's a completely personal decision.

As a developer I too spend all day reading and writing text on a screen and I can can tell you that there are so many things that go into what causes me eyestrain personally that I'm sure making any of it objective would be tough.

I have tons of problems with eyestrain when reading black-on-white text for long periods of time, so the kindle works great for me. I'm sure with a sufficiently configurable reading app I could tweak it until I could stand it, but I don't know too many PDF readers that allow you to start messing with background colors and the like.