Quoting Craig Sanders (cas@taz.net.au):
well formatted paragraphs are both easy and enjoyable to read - i.e. readable. bizarrely formatted paras with randomly strewn line-feeds and semi-colons are neither.
Just in case the point was unclear: 'Readability' relates to the content. If you're talking about presentation issues of any sort, including paragraph formatting, pagination, etc., then you are actually discussing legibility -- in contrast to readability. (Yes, I'm one of those tiresome people who think there's a difference between imply and infer, between less and fewer, et cetera. Awful people.)
i deliberately didn't use the word legible because illegible to me ^^^^& implies badly scrawled or scribbled or smudged or similar - none of which seemed appropriate for electronic text.