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

 

Some documents do not have the "Document Keywords" metadata set in the PDF.  If the "Document Keywords" 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.