If your PDF file was created from a born-digital document, such as a Microsoft Word file, it will almost always have embedded text. If your PDF file was created from scanned images (ie. JPG, PNG, TIFF) it does not have embedded text unless you have taken the additional step to OCR the image (or PDF file) and add that text to the PDF file.
There is more on working with PDF files in CONTENTdm.