Make existing PDF searchable ( OCR ) via command line / script I am looking for an offline scriptable tool that makes an existing PDF file searchable by running OCR on it, replacing the original non-searchable file with the searchable version, and can run unattended.

E.g., www.pdfscannerapp.com - does exactly what I need, but it's GUI only - not scriptable.

I am aware that Evernote makes PDF files searchable, but they remain searchable only when within Evernote.

I am not looking for perfect OCR, even a moderately acceptable OCR is fine, but I would prefer a small utility rather than a bulky software package.

(I am aware of a similar, but different question on AD: Looking for Software to Scan or Convert to Searchable and Signable PDF - however, I don't need to sign or fill PDFs, and my requirement is that the solution is scriptable)

EDIT: 

1) Several utilities allow structured text extraction, however in order to be extracted, the text must be there; I am mainly referring to PDFs that are wrapped bitmaps, as is the case with plain PDFs generated by scanners.

2) I am not necessarily looking for a free solution, and I would be more than happy to pay for a good utility that just does what I need, but I am not looking for bulky applications with a million features that include an OCR feature but whose cost does not justify buying them just for the OCR functionality.

3) As stated above, I am not looking for perfect OCR, just a moderately acceptable OCR. Unfortunately, in my experience, tesseract is really below that threshold. I define "moderately acceptable" an OCR that can, say, OCR an utility bill so that at least the account number (customer number) is recognized correctly.

EDIT: "scriptable" or "automatable", that is, able to be triggered automatically and run unattended without human input whatsoever.
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