попробуйте использовать Office Interop
private void _convetHTML2Doc( string FileNameUpload)
{
string filePath = Server.MapPath("~/htmlfile");
object missing = Type.Missing;
object FileName = @"D:\" + "\\" + FileNameUpload;
object readOnly = true;
m_word = new Application();
m_word.Documents.Open(ref FileName, ref missing, ref readOnly, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);
string newfilename = filePath + "\\" + FileNameUpload.Replace(".html", ".doc");
object o_newfilename = newfilename;
object o_encoding = Microsoft.Office.Core.MsoEncoding.msoEncodingUTF8;
object o_format = Microsoft.Office.Interop.Word.WdSaveFormat.wdFormatDocument;
object o_endings = Microsoft.Office.Interop.Word.WdLineEndingType.wdCRLF;
m_word.ActiveDocument.SaveAs(ref o_newfilename, ref o_format, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref o_encoding, ref missing, ref missing, ref o_endings, ref missing);
m_word.Quit(ref missing, ref missing, ref missing);
System.Runtime.InteropServices.Marshal.ReleaseComObject(m_word);
}