Great question Tammie! To do this, navigate to the desired client card. Then, in the top right, click on New. From the drop-down menu, click on Share Document if you intend the client to be able to see the document, or Internal Document if it is meant to be internal only and not shared. This will attach the document under the Documents tab on this client card, and if it's shared, will send to the client in an email. Click Here for a detailed guide! 

Thanks for your post, and if you run into anything else, leave us a comment!