Represents the "D" entry of the annotation additional-actions dictionary object.

It is an optional entry defined as part of the PDF 1.2 specification.

It contains an ActionElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 194, page 415.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 197, page 504.

The mouse-down action fires when the primary mouse button is pressed while the pointer is in the annotation's active area.