Abbyy Finereader (paid, Windows) is one of the best OCR programs, most of the books on archive.org are OCR'ed with Abbyy.
If the PDFs already have OCR text, calibre (GUI or CLI, Linux or Windows) can convert to .txt and many other formats. The recoll.org search engine will index PDF files that have OCR text.
If the PDFs already have OCR text, calibre (GUI or CLI, Linux or Windows) can convert to .txt and many other formats. The recoll.org search engine will index PDF files that have OCR text.