Word to PDF C#

//using Word = Microsoft.Office.Interop.Word;

Microsoft.Office.Interop.Word.Application _Word = new Microsoft.Office.Interop.Word.Application();
object _MissingValue = System.Reflection.Missing.Value;

private void simpleButton1_Click(object sender, EventArgs e)
{
    _Word.Visible = false;
    _Word.ScreenUpdating = false;

    // Cast as Object for word Open method
    string pathfile = @"D:\MyWord.doc";
    object filename = (object)pathfile;

    // Use the dummy value as a placeholder for optional arguments
    Word.Document doc = _Word.Documents.Open(ref filename, ref _MissingValue,
     ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue,
     ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue,
     ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue);
    doc.Activate();

    object outputFileName = System.IO.Path.ChangeExtension(pathfile, "pdf");
    object fileFormat = Word.WdSaveFormat.wdFormatPDF;

    // Save document into PDF Format
    doc.SaveAs(ref outputFileName,
     ref fileFormat, ref _MissingValue, ref _MissingValue,
     ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue,
     ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue,
     ref _MissingValue, ref _MissingValue, ref _MissingValue, ref _MissingValue);

    // Close the Word document, but leave the Word application open.
    // doc has to be cast to type _Document so that it will find the
    // correct Close method.    
    object saveChanges = Word.WdSaveOptions.wdDoNotSaveChanges;
    ((Word._Document)doc).Close(ref saveChanges, ref _MissingValue, ref _MissingValue);
    doc = null;

    // word has to be cast to type _Application so that it will find
    // the correct Quit method.
    ((Word._Application)_Word).Quit(ref _MissingValue, ref _MissingValue, ref _MissingValue);
    _Word = null;
}