To do this, first upload it to the File Library in your CMS site:
1. Go to Content > Content Management System
2. Hover over your site and click Manage Website
3. Click on the Content Manager tab
4. Click on File Library
5. Click Upload
6. Browse to find and select your file
7. Add a title and/or description
8. Upload the file
9. Select the file from the menu on the left hand side
10. Copy the URL of the file

Now, link to this URL within the content of your email:
1. Go to Communication > eMail
2. Hover over the Campaign Name and click Manage
3. Open up the Content of the email you wish to edit
4. Click on the Hyper Link tool
 - A new window opens, and the Link Type by default is "Add Your Own"
5. Paste the URL into the Hyper Link field
6. Add text to display in the Link Text field
7. Submit