Use document design to help readers locate information and understand the structure and meaning of your material. The design of a document has both an informational and a physical aspect--format and layout. Format refers to the arrangement of the document's content into standard subject areas such as introduction, theory, method and results, discussion, and conclusions sections. Format also refers to the general design of standard document elements such as tables and figures, as well as citations (parenthetical references, footnotes and bibliographies). Format conventions are usually widely followed but may vary from field to field.
Layout concerns the physical appearance and form of the document page and the document as a whole. Page layout exploits a variety of tools that include the use of headings, numbering systems, bullet and enumerated lists, white space, columns, margins, fonts, indentation, and justification. A document also has a total physical design. Covers, paper size and quality, colors, and two-sided printing may all be important considerations if you are producing a finished document rather than a manuscript.
Document format should be part of the initial consideration at the outline stage of a document. Page and document layout become increasingly important as your document nears its final form. But consider layout at the start for documents with complex layouts such as computer manuals and commercial documents.