Important ideas should stand out. You can use colour sometimes. Usually bold type is the easiest way to emphasize. Sometimes you can underline - but watch out, if the underlining is too close to the bottom of the letters, that can be confusing for new readers. It makes the letters seem to run together.
Pictures suggest the subject of the text.
Skilled readers pick up clues from the whole context, not just the text. Line drawings are the best for most documents - they're simple and clear, and they photocopy well. Try to use them in a simple way. Some readers prefer to have periods between each letter of the acronym, other prefer not to. You may also want to avoid isn't, they're, it's, haven't, won't, can't and other contractions, especially with new readers.