Make them as easy to read and carry as possible. Thus,
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If it’s a book manuscript, send it double-sided, and stapled or otherwise bound, rather than as a thick stack of single-sided pages.
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Print it single-spaced with wide margins, like a published work. There’s a reason that this is the way journals and books are published, and the reason isn’t just saving space — my sense is that such text is easier to read. Double-spaced text may be useful when you’re expecting lots of interlineated comments, but not otherwise.
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Use a proportionally spaced font, both for the text and the footnotes, rather than a typewriter font.
Maybe these are just my idiosyncratic reactions — please let me know if that’s so. But I suspect not; it seems to me that the traditional journal/book layout and format is inherently easier to read than the alternatives I recommenda gainst.