can u give me the code to upload a pdf file using file upload

your question is incomplete or having less information.which control you used.
i want to give you different ways:-
1]use file uploader controller with regular epression:-

<asp:FileUpload ID="flUpld" runat="server" />
                <asp:RegularExpressionValidator
                    id="RegularExpressionValidator1" runat="server"
                    ErrorMessage="Only PDF files are allowed!"
                    ValidationExpression="^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))(.pdf|.PDF)$"
                    ControlToValidate="flUpld" CssClass="text-red"></asp:RegularExpressionValidator>

2] Use ajax file uploader controller:-

<ajaxtoolkit:ajaxfileupload id="AjaxFileUpload1" xmlns:ajaxtoolkit="#unknown">
    ThrobberID="myThrobber"
    ContextKeys="fred"
    AllowedFileTypes="jpg,pdf"
    MaximumNumberOfFiles=10
    runat="server"/></ajaxtoolkit:ajaxfileupload>

3] or use javascript script to validate file is .pdf

<script type ="text/javascript">
    var validFilesTypes=["PDF","pdf"];
    function ValidateFile()
    {
      var file = document.getElementById("<%=FileUpload1.ClientID%>");
      var label = document.getElementById("<%=Label1.ClientID%>");
      var path = file.value;
      var ext=path.substring(path.lastIndexOf(".")+1,path.length).toLowerCase();
      var isValidFile = false;
      for (var i=0; i<validFilesTypes.length; i++)
      {
        if (ext==validFilesTypes[i])
        {
            isValidFile=true;
            break;
        }
      }
      if (!isValidFile)
      {
        label.style.color="red";
        label.innerHTML="Invalid File. Please upload a File with" +
         " extension:\n\n"+validFilesTypes.join(", ");
      }
      return isValidFile;
     }
</script>

this is simple file upload code you have to modify this code as per your requirement

protected void UploadButton_Click(object sender, EventArgs e)
{
    if(FileUploadControl.HasFile)
    {
        try
        {
            string filename = Path.GetFileName(FileUploadControl.FileName);
            FileUploadControl.SaveAs(Server.MapPath("~/") + filename);
            StatusLabel.Text = "Upload status: File uploaded!";
        }
        catch(Exception ex)
        {
            StatusLabel.Text = "Upload status: The file could not be uploaded. The following error occured: " + ex.Message;
        }
    }
}

or you can use server side validation and code is here. but i recommend to use client side validation and simple uploading code-
make change in below code as per your requirement:-

protected void btnUpload_Click(object sender, EventArgs e)
       {
           //Get path from web.config file to upload
           string FilePath = ConfigurationManager.AppSettings["FilePath"].ToString();
           bool blSucces = false;
           string filename = string.Empty;
           //To check whether file is selected or not to uplaod
           if (FileUploadToServer.HasFile)
           {
               try
               {
                   string[] allowdFile = { ".pdf" };
                   //Here we are allowing only pdf file so verifying selected file pdf or not
                   string FileExt = System.IO.Path.GetExtension(FileUploadToServer.PostedFile.FileName);
                   bool isValidFile = allowdFile.Contains(FileExt);
                   if (!isValidFile)
                   {
                       lblMsg.ForeColor = System.Drawing.Color.Red;
                       lblMsg.Text = "Please upload only pdf ";
                   }
                   else
                   {
                       // Get size of uploaded file, here restricting size of file
                       int FileSize = FileUploadToServer.PostedFile.ContentLength;
                       if (FileSize <= 1048576)//1048576 byte = 1MB
                       {
                           //Get file name of selected file
                           filename = Path.GetFileName(FileUploadToServer.FileName);
                           //Save selected file into specified location
                           FileUploadToServer.SaveAs(Server.MapPath(FilePath) + filename);
                           lblMsg.Text = "File upload successfully!";
                           blSucces = true;
                       }
                       else
                       {
                           lblMsg.Text = "Attachment file size should not be greater then 1 MB!";
                       }
                   }
               }
               catch (Exception ex)
               {
                   lblMsg.Text = "Error occurred while uploading a file: " + ex.Message;
               }
           }
           else
           {
               lblMsg.Text ="Please select a file to upload.";
           }
           //Store file details into database
           if (blSucces)
           {
               Updatefileinfo(filename, FilePath + filename);
           }
       }