Selecting the "Document Title" rename option uses the document title metadata embedded in the PDF file in the renamed file name.

 

Some documents do not have the "Document Title" metadata set in the PDF.  If the "Document Title" metadata is empty, a "-" character is substituted in the renamed file.

 

You can view the PDF metadata fields by opening the PDF in Adobe Acrobat Reader DC and viewing the "Document Properties" under the "File" menu.