I have the original as hardcover, but also wanted to be able to read on the go on the iPad, so I downloaded the ebook.
It's a scan with OCR. Unfortunately the OCR is bad on many parts of the page - random occurences of %, ' and other characters are common. Also some words are totally unreadable.
Overall, I still read sometimes on the iPad, but having to go back to the hardcover if I'm totally lost.
Just because there is a PDF (or ePub etc.) of a book exists doesn't mean there is an official ebook. Some pirates scan books and either upload those each-page-is-one-image pdfs or (worse) they OCR these images andc upload that text.