While PDF is a great tech demo in practice,unless one is using it for short documents,it doesnt work very well.That's why google did what it did with doc,render the pdf as an image them put a transparent text over it. It's a better approach if one has good servers. because frankly one cant expect the browser to behave like acrobat reader and be as performant as native applications when trying to render 20MB pdfs (and a lot of books are that big,this is not an rare use case).