We apologize for the delay in response to your query.
As you have mentioned that the PDF document has hand written signature on it. So I assume that PDF document was initially scanned and then saved as .pdf file?
What was the PDF producer to create the .pdf file? Also, when the document was scanned or "Scanned to pdf" it doesn't contain actual text it just had the image in it.
I believe you work on Adobe Reader DC which doesn't have the ability to run the OCR (optical character recognition).
poor quality scans, such as those where the scanned page is skewed, or there are smudges or marks on the page, are very difficult for ExportPDF to convert. When possible, re-scan those documents to produce a higher-quality scan.