I used to have a pdf version of each chapter, but no one used them. I moved to having a pdf of every volume (5 chapters) and a couple of people have accessed them, but still nothing revelation-inducing. I think its still dictated by demographics - the majority of my audience aren't going to have e-readers which are still the toys of the white-middle-class professionals.
The other issue with pdf and e-book readers is how they handle images. Since I don't *have* a reader I don't know how/if they do. This wouldn't be an issue if the only image I had was the cover, but my narrative diverges into webcomic territory at various points. This proves causes layout issues with things that use standard pages (like normal books!) and until I know how e-readers deal with that my pdf remain text only (with illustrated scenes replaced with text, totally weakening their intended impact!)]
But I keep them up there just in case.