If its a scanned PDF (essentially a collection of 1 image per page), there would need to be an OCR step to get some text out first. Tesseract would work for this.

Once that's done, you have all the options available to perform that search. But I don't know of a search tool that does the OCR for you. I did read a blog post of someone uploading PDFs to google drive (they OCR them on upload) as an easy way to do this.