Save pdf file with username (iTextSharp)

I want to allow the user to enter their own filename, as well as save the dialog and file stream (Example: Stream s = File.Open(sfdPdf.FileName, FileMode.CreateNew)

Here is my code:

    private void btnSave_Click(object sender, EventArgs e)
    {

        System.Drawing.Rectangle bounds = this.Bounds;
        using (Bitmap bitmap = new Bitmap(bounds.Width, bounds.Height))
        {
            using (Graphics graphics = Graphics.FromImage(bitmap))
            {
                graphics.CopyFromScreen(new Point(bounds.Left, bounds.Top), Point.Empty, bounds.Size);
            }
            bitmap.Save("Image.jpeg", ImageFormat.Jpeg);
        }

        Document doc = new Document(PageSize.LETTER, bounds.Left, bounds.Right, bounds.Top, bounds.Bottom);
        PdfWriter wri = PdfWriter.GetInstance(doc, new FileStream("ImageTest.pdf", FileMode.Create));
        doc.Open();
        iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance("Image.jpeg");
        doc.Add(image);
        doc.Close();
}

      

        
        
        
      

    

I want the "ImageTest.pdf" part to be named as user with pdf extension (and .pdf filetype).

PdfWriter writer = PdfWriter.GetInstance(doc, new FileStream("ImageTest.pdf", FileMode.Create));

Can anyone please help or does anyone have a better solution for my problem? I want to take a screenshot of my window form and export the image to pdf file under username

EDIT: With saveFileDialog (after bitmap.save) - Receive error "Format error: not PDF or corrupted".

SaveFileDialog sfd = new SaveFileDialog();
sfd.Filter = "Pdf File |*.pdf";
if (sfd.ShowDialog() == DialogResult.OK)
{
    using (Stream s = File.Open(sfd.FileName, FileMode.CreateNew))
    {
        Document doc = new Document(PageSize.LETTER, bounds.Left, bounds.Right, bounds.Top, bounds.Bottom);
        PdfWriter wri = PdfWriter.GetInstance(doc, new FileStream("ImageTest.pdf", FileMode.Create));
        doc.Open();
        iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance("Image.jpeg");
        doc.Add(image);
        doc.Close();
        s.Close();
        s.Dispose();
    }               
}