This class represents the pop-up annotation. This is definitively detailed in:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 183, page 406.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 186, page 492.

This class is always an indirect object because MarkupAnnotationElement.EntryPopup requires it to be so.

System.Object
    WebSupergoo.ABCpdf14.Elements.Element
       WebSupergoo.ABCpdf14.Elements.AnnotationElement
          WebSupergoo.ABCpdf14.Elements.PopUpAnnotationElement

A pop-up annotation presents the text of a markup annotation in a floating window. It carries no content of its own; text and author come from the parent markup annotation referenced by the Parent entry.

The Open entry records whether the pop-up window is currently open, allowing this state to be persisted across document saves.