Represents the "NS" entry of the namespace dictionary object.

It is a required entry defined as part of the PDF 2.0 specification.

It contains a string representing a PDF string object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 356, page 725.

The NS entry holds the namespace URI as a text string, uniquely identifying the namespace and distinguishing its element types from those of other namespaces.