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.
For scanned documents, PDF2XL OCR uses OCR - Optical Character Recognition.
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.