Online documents are harder for an audience to read than when viewing something from paper. According to (Parker 2003 pp. 270-275) there are a few main reasons for this. The first is that when you are reading off a screen your reading from a projected light which is hard for the eye to focus on for a long time and causes eye fatigue. When reading from a printed piece of paper you are actually looking at the reflected light from the natural light surrounding us, bouncing off the colours of the page which is much easier to absorb.
There is also a much more limited space for text as when your reading from a page its generally a vertical layout however on a screen page it must be horizontal. You don’t want your audience to have to constantly scroll up and down to follow the page, however if you try to fit an entire page on a screen it becomes far too small to comfortably read from. Multi-column layouts are also something that must be avoided as again this would require the reader to scroll up and down making it easy to loose track of where you are or to quickly skim back to clarify certain points.
Parker, Roger C 2003, Designing Documents for web distribution, Paraglyph Press, Scottsdale