Represents the "D" entry of the thread action object.

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

This property may contain one of three different types:.

1) An integer representing a PDF numeric object.

2) A string representing a PDF string object.

3) A ThreadElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 205, page 423.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 209, page 514.