If you have a MS 2007(with XPS and PDF addon) or MS2010, you can directly save the doc/docx files to pdf using the "SaveAs" method.
object filename = Path.GetTempPath() + @"\test.pdf";
object fileFormat = WdSaveFormat.wdFormatPDF;
oDoc.SaveAs(ref filename, ref fileFormat, ref oMissing, ref oMissing, ref oMissing, ref oMissing,
ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing,
ref oMissing, ref oMissing, ref oMissing);
object save = false;
oDoc.Close(ref save, oMissing, oMissing);