Word.Document document = axWebBrowser.Document as Word.Document;
if(document != null)
{
    object oMissing = System.Reflection.Missing.Value;
    object noReset = true;
    object password = @"qwerty";
    document.Protect(Word.WdProtectionType.wdAllowOnlyReading, 
        ref noReset,
        ref password,
        ref oMissing,
        ref oMissing); 
    document.Saved = true;
}