There are two distinct types of PDF documents: regular and scanned.
Regular documents are usually created using Adobe Acrobat, or a special printer driver that prints into a PDF file. These files contain actual text.
Scanned documents, on the other hand, are usually created by scanning a physical document, and therefore contain only an image of the text.
Converting scanned documents requires you to upgrade your product to PDF2XL OCR, which contains the OCR (Optical Character Recognition) module.
The OCR module attempts to read the text inside the images so it can be converted to Excel properly.
When you open a scanned document, PDF2XL OCR will usually recognize it and suggest to turn on OCR Mode. A message box will be displayed, saying that the document is scanned and that it will be displayed in OCR Mode, and allow you to check a "Don't ask me again" box, which will make any scanned document you open in the future use OCR Mode automatically and without notifying you.