Private _flag
_flag: NpdfAnnotationFlag.Invisible | NpdfAnnotationFlag.Hidden | NpdfAnnotationFlag.Print | NpdfAnnotationFlag.NoZoom | NpdfAnnotationFlag.NoRotate | NpdfAnnotationFlag.NoView | NpdfAnnotationFlag.ReadOnly | NpdfAnnotationFlag.Locked | NpdfAnnotationFlag.ToggleNoView | NpdfAnnotationFlag.LockedContents