One other, maybe less obvious, road is to ask the professor for the source material from which the PDF was generated. You might have an easier way starting from there. And your professor might be willing to supply you with the material with the lure of getting an ebook compatible version of the text in exchange. Even if the original material are individual scans, you are better of starting with those images for OCR, than with PDF files (which is, apart from its multipage capabilities, fundamentally unsuitable for scanned material)