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

It is an optional entry defined as part of the PDF 2.0 specification.

It contains a FileSpecificationElement.

For definitive details see:.

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

The Schema entry references an associated file containing a schema for the namespace. The schema formally defines element types, permitted attributes, and their relationships.