Represents the "URL" entry of the developer extensions
It is an optional entry defined as part of the PDF 2.0
It contains a string representing a PDF string object.
The specification defines this as exensible which means that it
may contain entries in addition to those defined in the
In fact almost all objects are extensible but the fact that it
is explicitly mentioned, implies that extra entries should be
regarded as normal rather than exceptional.
For further details see
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: Annex
E, page 673..
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 50,
Adobe Supplement to the ISO 32000, BaseVersion: 1.7,
ExtensionLevel: 3; Table: 3.25b, page 25.
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 49, page 140.