Problem with many PDFs these days is that they don't contain text, only a picture of the text page. PDF to text converters can only extract from text-pdf files. If you scan a page into pdf, or into e-mail, you get a pdf with an embedded picture of the scanned page.
If you have a picture with text, you can use an OCR program instead. A bit more cumbersome, but ok if there's a lot of text and the quality is good.