Each citation record in your EndNote 20 library may have multiple files attached to them.

To attach items:

1. Select an item in your EndNote 20 library by clicking on it, and click + Attach File in the right-hand panel citation preview.
2. Using the dialogue box, navigate to the attachment's file location.
3. Select the attachment and click Open.
4. The attachment will now appear above the + Attach File button, in the citation preview.
5. Select File > Save (or CTRL+S).

OR 

1. Drag the file that you wish to attach from your file explorer.
2. Drop the file on the reference you wish to attach it to.
3. Save.