I’ve just had a very bad experience, because I relied rather uncritically on a volume that I found on Google books. It’s a warning, and I doubt I shall forget it in a hurry.
I have someone out in the Middle East transcribing the Arabic from Erpenius’ 1625 edition of the 13th century Coptic historian, al-Makin. Of course I got a copy from google books and sent it off, and thought no more about it.
The text is 300 pages. It turns out that various pages are missing, others appear out of order, or several times. Of course the transcriber was chosen for their Arabic skills, and, although they’ve done their best, have been utterly confused by this. Worse yet, they live in a region where internet access is poor, so downloads are very slow.
I have had to spend the entire evening working on the Erpenius PDF in Adobe Acrobat 9 Pro; indicating, page by page, whether the page should be included or not; marking up individual pages with red crossings out; inserting missing pages from another copy.
I’ve had to do this so that the transcribe can go through their transcription, in the order of the original defective PDF, and find the material in the right places.
It’s a hideous job.
Moral: never rely on a Google books PDF. Take the time and just go through it and collate it. It will take 15 minutes at most, and it will save you a world of frustration.