Add rectangle annotation to a PDF in Xamarin

To add a rectangle annotation to a PDF Document page.

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

// Create a rectangle
Square sq = Square.Create( doc, new Rect(500,200, 580, 300 ) );
sq.SetColor(new ColorPt(1, 0, 0), 3);
sq.SetInteriorColor(new ColorPt(0, 1, 1), 3);
double[] dash = new double[2];
dash[0]=4;dash[1]=2;
sq.SetBorderStyle(new Annot.BorderStyle( Annot.BorderStyle.Style.e_dashed, 6, 0, 0, dash ) );
sq.SetPadding( new Rect(4,4,4,4) );
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).