Represents the "ExData" entry of the markup annotation object.

It is an optional entry defined as part of the PDF 1.7 specification.

It contains an ExternalDataElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 170, page 393.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 8.21, page 38.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 332, page 695.

The external data dictionary links this annotation to data that cannot be fully represented within its standard entries, such as a 3D node for a projection annotation.