To add a document to a massage you need to upload a document first.

Afterwards you can add the document to your messages by following those steps:

Select a job from the dashboard. You can see all the job details and applicants for this position. 

Select the applicant to whom you want to send a message. Click on the button "message" and choose from the options: Invite, Dismiss, General and personality test. After you chose one of the options a new window opens and you can write a message to the applicant or choose from one of your templates. 

In the section "add document" you can choose a document from your already uploaded documents, add it to the message and send it.