They will not scroll down unless you have hooked them with what they see at their first glance. They will never, ever, scroll laterally. They want quick, straight information. They lose the thought while the next page loads. The eye can't track as smoothly when scrolling.
Perception is more work on a self-luminous source.
The result is that people read differently to conserve cognitive effort. Other reasons have to do with why the reader is there, how they found your information, and how critically they are viewing it. More often than not web-readers are using the web more like a reference source than like expository text. More often than not they have no reason to suppose you know what you're talking about. Unlike the web, it usually takes at least some qualifications to get your words into printed material.