Represents the "AU" entry of the source information dictionary object.

It is a required entry defined as part of the PDF 1.0 specification.

This property may contain one of two different types:.

1) A string representing a PDF string object.

This is encoded as a string using single byte ASCII encoding.

2) An UrlAliasElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 355, page 623.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 391, page 803.