There are two ways to attach documents to clients:
1. BCC "firstname.lastname@example.org"
In a new email or in a reply to the client, within your 3rd party email system (Gmail, Outlook, Etc), BCC "email@example.com". This will automatically save the attachments/documents to the "Document" page of the client. Along with saving the email's subject and body to the "Contact Activity Log".
2. Or go to the "Document" section on the navigation bar and upload the document there. View the article: Uploading Documents