Try to copy some text from your PDF file(s) into a notepad or another text editor. Probably, you could not select and copy any text because your document is not machine-readable text. It requires Optical character recognition (OCR) which is a conversion of images of typed, handwritten, or printed text into machine-encoded text. Therefore no words are extracted from your document.
You need to use an OCR system for your document first. You can find many OCR converters on the Web, e.g. OnlineOCR, NewOCR, Convertio, etc. Only don't forget that the OCR method is usually not 100% accurate. The accuracy mostly depends on the quality of your files (PDF, images) and it may produce broken words even for good quality images.
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