Why do some résumés and letters grab readers' attention? Why are some forms hard to read and some instruction manuals difficult to understand? What makes a report clear? Often the answer lies in the the document's design - how the writer has integrated text, visual aids (graphical aids, photographs, and illustrations), and type to engage the reader.
The bottom line is that, as we've said before, workplace audiences generally skim, rather than read closely. Document Design (like Style) helps audiences get what they need from your document more easily - good document design makes information visually accessible.