One of the advanced features available to paid subscribers is the ability to store documents in your account's GeniusDrive. This feature allows you to add documents and files as attachments to sign ups and/or emails.
To manage files, login to your account and click Tools on the left side of your account Sign Ups page. Choose GeniusDrive and upload your document(s). You can then go back to your Sign Ups account page and click on the pencil icon to the right of the sign up for which you want to attach a document. On the Design tab, locate the Related Files area on the sign up preview. Click the paper clip icon to select files stored in your GeniusDrive. It may also be helpful to know that you can attach documents to an email when using the Messages area of the account to send emails.
Learn more about attaching files to your sign ups and email messages with this helpful Genius Hack.