Your PDF sometimes may contain scanned images that do not contain any searchable text, perhaps because it has not been processed by an OCR application. To identify such PDF files, select the List empty files item from the Tools menu.
This opens up the List Empty PDF Files window.
On the right is the list of PDF files where no text was indexed. On the left is the list of actions you can perform using that list of files.
Once you have selected one or more actions, click on the Start button.