This study aims to clarify the factors behind readability of paper documents; the clarification is intended to yield good guidelines for realizing truly readable Electronic Paper. Proofing tasks were prepared on various reading conditions; display area was varied from 1/2 page to 4 pages on the screens. Performance is shown to increase with the number of pages simultaneously provided. This result agrees to our general impression that we generally feel that it is difficult to complete proofing tasks on a computer screen, which usually provides less than one page.