private void button2_Click(object sender, EventArgs e)

        {

            MessageBox.Show(GetNoOfPagesPDF(textBox1.Text).ToString());

        }

private void button1_Click(object sender, EventArgs e)

        {

            if (openFileDialog1.ShowDialog() == DialogResult.OK)

            {

                textBox1.Text = openFileDialog1.FileName;

            }

        }

public static int GetNoOfPagesPDF(string FileName)

        {

            int result = 0;

            FileStream fs = new FileStream(FileName, FileMode.Open, FileAccess.Read);

            StreamReader r = new StreamReader(fs);

            string pdfText = r.ReadToEnd();

            System.Text.RegularExpressions.Regex regx = new Regex(@"/Type\s*/Page[^s]");

            System.Text.RegularExpressions.MatchCollection matches = regx.Matches(pdfText);

            result = matches.Count;

            return result;

        }