Unfortunately ResultsMail does not currently support the use of attachments.
The reason for this is that attachments typically make the total email size very large. The larger the email that is delivered the more likely it is to be blocked, cause clogged inboxes, etc. Therefore, for the sake of increased deliverability we do not support attachments.
Instead you would need to place the document up on your website and create a link within your ResultsMail email campaign to have people download the document.
The added benefit of this approach is that the recipient will only need to download the document if they are interested in it and you will be able to track via the ResultsMail reports how many recipients have clicked through to retrieve the document.