Why my textbox in PDF can not show?

Hi every one,

I want to insert a textbox in my PDF document, I use the below code, but it when I open the PDF document, it can not show itself, but when I click the blank part of the PDF document, it shows itself. can anyone tell me why? I use a PDF comonent Spire.PDF. The code is shown as below:

using System.Text;
using Spire.Pdf;
using System.Drawing;
using Spire.Pdf.Fields;
using Spire.Pdf.Graphics;

namespace PDFtextbox
{
    class Program
    {
        static void Main(string[] args)
        {
            PdfDocument pdfDoc = new PdfDocument();
            pdfDoc.LoadFromFile(@"C:\Spire.PDF\Demos\Data\smile.pdf");
            PdfPageBase page = pdfDoc.Pages[0];
            PdfTextBoxField textField = new PdfTextBoxField(page, "textBox1");
            float x = page.Canvas.ClientSize.Width / 5;
            float y =(float) (page.Canvas.ClientSize.Height / 1.9);
            textField.Bounds = new RectangleF(x, y, 3 * x, 40);
            textField.BorderWidth = 1f;
            textField.BorderStyle = PdfBorderStyle.Solid;
            textField.Text = "Smile to the world, the world will smile back to you";
            textField.BorderColor = new Spire.Pdf.Graphics.PdfRGBColor(Color.Olive);
            textField.BackColor = new PdfRGBColor(Color.LightYellow);
            textField.Font = new PdfFont(PdfFontFamily.TimesRoman, 12f, PdfFontStyle.Regular);
            textField.ForeColor = new PdfRGBColor(Color.Green);
            pdfDoc.SaveToFile(@"result.pdf");
            System.Diagnostics.Process.Start(@"result.pdf");
        }
    }
}

Thanks in advance!

Never give up! Smile to the world!
http://excelcsharp.blog.com/