var page = pdfViewer1.CurrentPage;
float x1, y1, x2, y2;
x1 = 100; y1 = 400;
x2 = 400; y2 = 300;
var line = PdfPathObject.Create(FillModes.None, true);
line.Path.Add(new FS_PATHPOINTF(x1, y1, PathPointFlags.MoveTo));
line.Path.Add(new FS_PATHPOINTF(x2, y2, PathPointFlags.LineTo));
float[] dash = { 10, 5, 2, 5 };
line.LineWidth = 3.0f;
line.StrokeColor = FS_COLOR.Red;
line.SetDashArray(dash);
page.PageObjects.Add(line);
page.GenerateContent();
page.ReloadPage();
pdfViewer1.Invalidate();