You can add an attachment to an email campaign either when you are creating a campaign or when editing a campaign.
When you are in the WYSIWYG email editor, click the "Add Attachment" button - a pop up screen will appear. Choose your file, click the 'OK' button and the attachment will be added to your campaign.
When you are in the Drag-N-Drop email editor, click the 'Attach' link on the left side of the screen. This will reveal a green 'Add Attachment' button - click on it and choose your file. The attachment will be added to your campaign.
A Couple of Notes:
The file limit size for a campaign email attachment is 1MB, but about 250k is recommended.
Be sure that the filename and content of the attached file are not spammy