'Retrieves the PDF file\'s document information dictionary, if it exists.
Note that some PDF files use metadata streams instead of docinfo
dictionaries, and these metadata streams will not be accessed by this
function.
:return: the document information of this PDF file
:rtype: :class:`DocumentInformation<pdf.DocumentInformation>` or ``None`` if none exists.'

def getDocumentInfo(self):

    if ('/Info' not in self.trailer):
        return None
    obj = self.trailer['/Info']
    retval = DocumentInformation()
    retval.update(obj)
    return retval
