To add a rectangle annotation to a PDF Document page.

doc = PDFDoc.new(filename)
page = doc.GetPage(1)

# Create a rectangle
sq = Square.Create( doc.GetSDFDoc(), Rect.new(500,200, 580, 300 ) )
sq.SetColor(ColorPt.new(1, 0, 0), 3)
sq.SetInteriorColor(ColorPt.new(0, 1, 1), 3)
sq.SetBorderStyle( BorderStyle.new( BorderStyle::E_dashed, 6, 0, 0, [4, 2] ) )
sq.SetPadding( 4 )
sq.RefreshAppearance()
page.AnnotPushBack( sq )

Add or edit PDF annotations sample
Full code sample which shows how to add or edit PDF annotations (e.g. hyperlink, intra-document link, stamp, rubber stamp, file attachment, sound, text, free-text, line, circle, square, polygon, polyline, rectangle, squiggly, caret, and ink).