PdfReader reader = new PdfReader(@"C:\WorkSpace\1.pdf");
int count = reader.NumberOfPages;

TextField fieldDate = new TextField(stamp.Writer, new iTextSharp.text.Rectangle(105, 100, 240, 125), "date");
fieldDate.BackgroundColor= BaseColor.WHITE;fieldDate.BorderWidth= 1;
fieldDate.BorderColor= BaseColor.BLACK;fieldDate.BorderStyle= 4;
fieldDate.FontSize = 11f;

Chunk cname = new Chunk("Date:", FontFactory.GetFont("Futura", 16f,new BaseColor(170,64,0)));
Phrase pname = new Phrase(cname);
PdfContentByte over = stamp.GetOverContent(count);
ColumnText.ShowTextAligned(over, Element.ALIGN_CENTER, pname, 400, 420, 0);

public static void AddTextField()
{
   PdfReader reader = new PdfReader(@"C:\WorkSpace\1.pdf");
      
   FileStream out1 = new FileStream(@"C:\WorkSpace\2.pdf", FileMode.Create, FileAccess.Write);

   PdfStamper stamp = new PdfStamper(reader, out1);
      //获得pdf总页数
   int count = reader.NumberOfPages;

   TextField fieldDate = new TextField(stamp.Writer, new iTextSharp.text.Rectangle(105, 100, 240, 125), "date");
   fieldDate.BackgroundColor = BaseColor.WHITE;
   fieldDate.BorderWidth = 1;
   fieldDate.BorderColor = BaseColor.BLACK;
   fieldDate.BorderStyle = 4;
   fieldDate.FontSize = 11f;

   TextField fieldSign = new TextField(stamp.Writer, new iTextSharp.text.Rectangle(430, 100, 530, 125), "sign");
   fieldSign.BackgroundColor = BaseColor.WHITE;
   fieldSign.BorderWidth = 1;
   fieldSign.BorderColor = BaseColor.BLACK;
   fieldSign.BorderStyle = 4;
   fieldSign.FontSize = 11f;

   Chunk cname = new Chunk("Date:", FontFactory.GetFont("Futura", 16f,new BaseColor(170,64,0)));
   Chunk ctitle = new Chunk("User Sign:", FontFactory.GetFont("Futura", 16f, new BaseColor(0, 128, 128)));
   Phrase pname = new Phrase(cname);
   Phrase ptitle = new Phrase(ctitle);

   //PdfContentBye类，用来设置图像和文本的绝对位置
   PdfContentByte over = stamp.GetOverContent(count);
   ColumnText.ShowTextAligned(over, Element.ALIGN_CENTER, pname, 400, 420, 0);
   ColumnText.ShowTextAligned(over, Element.ALIGN_CENTER, ptitle, 400, 350, 0);

   stamp.AddAnnotation(fieldDate.GetTextField(), count);
   stamp.AddAnnotation(fieldSign.GetTextField(), count);

   stamp.FormFlattening = true; 

   stamp.Close();
}