Make existing PDF searchable ( OCR ) via command line / script
What you want is Tesseract OCR. It's an open source OCR that is maintained by Google and supports a variety of platforms.  It also has a native command line interface.  It's exactly what you're looking for and available from the Mac ports project as well as homebrew. 

Project Home: https://github.com/tesseract-ocr

How to install on OS X: http://blog.matt-swain.com/post/26419042500/installing-tesseract-ocr-on-mac-os-x-lion

Usage Example: tesseract -l eng input.pdf output