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PDFs are not all created alike. In particular, when they start life a scanned images, OCR needs to run in order to overly text. And the software creating this text layer may not take the trouble to create separate regions for each word/letter. Without those, it’s not possible for the software to know where the words are.
You might try a PDF viewer on a computer to see if it has the same problem, and if so, there are probably tools that can fix this issue.
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