I used a service called Online OCR to convert some pdf files into an editable form. It is normally easy to extract the text from pdfs made with newer software, but for some files optical character recognition is necessary.
It worked flawlessly. The site offers limited free access and then charges a few cents per page. It also promises an extra 50 free pages if I place a link to Online OCR in some well-visited blog, so here is the link: http://www.onlineocr.net
I and satisfied and impressed, so I don't mind running an advertisement for them in this blog. Whether they will consider it well-visited is another story. Enjoy!