Easiest/cheapest way to OCR an existing PDF that lacks a text layer?

I have photo-scanned one of my most-used reference works, Watching the Skies . The aim was to allow me to do searches instead of using the (somewhat odd) index. I copied the images off my iphone and used Preview on my mac to produce a single PDF with all of the pages. The result is surprisingly good. So now I have a huge document of fairly readable scanned images. But there is no "text layer" that I can search. Is there a way to do that within Preview, or some other tool I can use?