При создании TextField
установите его свойство Rotation
:
PdfReader reader = new PdfReader(file1);
using (FileStream fs = new FileStream(file2, FileMode.Create, FileAccess.Write, FileShare.None))
{
using (PdfStamper stamper = new PdfStamper(reader, fs))
{
TextField tf = new TextField(stamper.Writer, new iTextSharp.text.Rectangle(0, 0, 100, 300), "Vertical");
//Change the orientation of the text
tf.Rotation = 90;
stamper.AddAnnotation(tf.GetTextField(), 1);
}
}