onAnnot

Check for an annotation containing the specified point.
onAnnot([in] int page, [in] double x, [in] double y, [out, retval] LONG *annotHandle)
If the specified coordinates on the specified page are inside (over) an annotation, this function returns an annotation handle. If the coordinates are not inside an annotation, it returns 0.

The coordinates are in the PDF coordinate space.

VB6:
Private Sub viewer_MouseDown2(ByVal button As Integer, ByVal shift As Integer, _ ByVal page As Long, ByVal x As Double, ByVal y As Double) annot = viewer.onAnnot(page, x, y) If annot <> 0 Then annotType = viewer.getAnnotType(annot) If annotType = "Text" Then content = viewer.getAnnotContent(annot) ' ... display the popup text annotation content ... End If End If End Sub
getAnnotType
getAnnotContent