var annotation = new PdfInkAnnotation(page);
annotation.Color = new FS_COLOR(1f, color.r, color.g, color.b);
annotation.LineStyle = new PdfBorderStyle();
annotation.LineStyle.Style = BorderStyles.Solid;
annotation.LineStyle.Width = 5;

var pdfLinePoint = new PdfLinePointCollection<PdfInkAnnotation>();
annotation.InkList = new PdfInkPointCollection();

for (var j = 0; j < positions.Count; j++)
{
pdfLinePoint.Add(new FS_POINTF(x, y));
}

annotation.InkList.Add(pdfLinePoint);
annotation.RegenerateAppearances();
page.Annots.Add(annotation);