To set an annotation style property such as color, interior color, border style, padding, etc to an annotation.

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

Circle circle= Circle.Create(doc, new Rect( 100, 100, 200, 200 ));
circle.SetColor(new ColorPt(0, 1, 0), 3);
circle.SetInteriorColor(new ColorPt(0, 0, 1), 3);
double[] dash = new double[2];
dash[0]=2;dash[1]=4;
circle.SetBorderStyle(new Annot.BorderStyle( Annot.BorderStyle.Style.e_dashed, 3, 0, 0, dash ) );
circle.SetPadding( new Rect(2,2,2,2) );
circle.RefreshAppearance();
page.AnnotPushBack(circle);

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, highlight, squiggly, caret, and ink).