In the PDF Import dialog, Under Options,  you can set the conversion options (default is "Perform Intelligent Recognition").
If the text in the PDF-file is not converted into text entities then your pdf-file included only filled areas and no text characters. The text in the PDF must not be "merged" with or part of an image, in order to be identified as text: Verify the options selected when creating the PDF file, in order to avoid loss of functionality.

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