Problems with PDF file

I'm showing a PDF file directly in the browser. In Google Chrome, it works perfectly, however, if you use FireFox, the PDF is as shown below.

Did you have to put some extra config for Firefox? The PDF file is ok if it opens directly in the folder.

Controller code

public ActionResult Boleto(string id)
{
    var arquivoPDF = Path.Combine(MegaBoleto.PastaPDF(), String.Format("{0}.{1}", id, "pdf"));
    if (System.IO.File.Exists(arquivoPDF))
    {
        return File(arquivoPDF, "application/pdf");
    }
    return View();
}