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Tesseract is a very good OCR engine: https://github.com/tesseract-ocr/tesseract
The project has been launched by HP Labs and is now continued and sponsored by Google (for Google Books !). It is released under the Apache license, and it runs on Linux. It uses Tiff or PNGs files ; for PDFs, you will need to convert to one of these formats. I suppose that there is no binding so you should invoke this software as a subprogram...