The design of a document strongly affects how effective it is. Chapter 14 of Anderson's Technical Writing text covers effective document design and gives some useful examples of designing to achieve specific objectives.
What I took away from this chapter was the idea that a document should be designed to support a reader's purpose and make intuitive sense to them. Using proximity, alignment, and text emphasis can greatly enhance a document's effectiveness. Placing related elements near one another and in proximity to one another, you can help readers determine a level of connection between the elements. Keeping a format consistent for sections of related content helps the reader more easily access information once they understand the precedent.
Using emphasis can also help a reader break down concepts and understand larger context versus smaller context. Overall, this chapter basically shows us why some documents are easy to read and some are not. Our goal is to use this information to make our own work more reader centered and more effective.