Javascript – How does one open a pdf in a new tab using C# asp.net

i can read the pdf and open it in the current tab however everything i have tried to open the pdf in a new tab is not working. I added javascript as seen in the commented out sections and yet it always opens in the same tab. Is it something i am missing or am i overthinking it

protected void bttnpdf_Click(object sender, EventArgs e)
{
    string fileName = null;
    fileName = "testPdf1.pdf";

    if (fileName != null)
    {
        string path = @"c:TestCodeTestPdfs" + fileName;
        if (File.Exists(path))
        {
            WebClient User = new WebClient();
            Byte[] FileBuffer = User.DownloadData(path);
            if (FileBuffer != null)
            {
                ScriptManager.RegisterStartupScript(Page, typeof(Page), "OpenWindow", "window.open('" + FileBuffer + "');", true);
                Response.ContentType = "application/pdf";
                Response.AddHeader("content-length", FileBuffer.Length.ToString());
                Response.BinaryWrite(FileBuffer);
            //string script = string.Format("window.open('{0}','_blank');", FileBuffer);
            //ScriptManager.RegisterStartupScript(this, GetType(),
            //                      "ServerControlScript", script, true);
            //bttnpdf.SetClientScript(string.Format("window.open('{0}','_blank');", FileBuffer), false);
            }
        }
    }
}