It is not the best practice you send attachments as part of your email campaigns. Your emails may end up in the spam folder of your subscribers and you will be forcing them to store the file in their mailboxes.
A better solution is to upload the document to your Sitefinity website and then provide the link to that document using the Document link
widget. That way only the subscribers that really want to access the document will be able to do so via the link you provided.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items