This study aims to clarify the factors behind the readability of paper documents; the clarification is intended to yield good guidelines for realizing truly readable Electronic Paper. Proofreading tasks were conducted under various reading conditions; display area was varied from 1/2 page to 4 pages on the screens. Proofreading performance is shown to increase with the number of pages simultaneously provided. This result agrees with our general impression that it is difficult to complete proofreading tasks on computer screens, which usually provide less than one page. The disadvantages of small display areas were confirmed regardless of the subjects' ages. Helpful guidelines for designing readable Electronic Paper were suggested by our study.