I’m currently proofreading a previously-published manuscript (not by me).
The new publisher managed to get ahold of a pdf that may be the final version of the book as previously published — or may be a “close to final” version with some typos. Plus, the fact that the new publisher has had to re-typeset the text may have introduced errors as well, such as missed italics, failure to recognize end-of-line hyphens properly, and who knows what else.
My method is that I with one eye I am reading the new pages on paper, and with another eye I am looking at my computer screen where I have an actual scan of the previous edition of the book. The pages do not match one for one, since they use different fonts, but they’re pretty close, which is a relief.
So far I found a good share of typos — enough to make the work satisfying, but not enough to cause any sense of dismay at all. I even found one typo (an extra space) that appeared in both the old and new edition.
The book is 300+ pages. I’m on page 26 now, after an hour or two.
Oh! Also, for fun, I am using actually proofreaders marks as I mark the mistakes. Yet I do not know these marks, so I have several manuals of style open to help me. Ironically, I’d saved a sheet with these marks for years, but never studied them, and the sheet is in storage.
Update: Sunday. I’m going frigging crosseyed from this. I’m on page 134 now.