To my mind this makes no sense.
A PDF file (essentially a photograph) is selectable, searchable and editable via Preview and Adobe Reader inter alia.
Why would it need to be OCRed when it's already manipulable?
To begin with your final exhortation: because it
isn't already manipulable. Elaborating on Joboy's previous post: it seems to me that you may have missed the following: PDFs may contain both text and images. Depending on the PDF(author)'s settings, you may select and copy those elements. JoBoy is referring to
image elements which depict text, but can only be selected and copied as image. It's those text images he'd like to convert to editable text.
