Sign in to your client's account from the dashboard. They must be active clients in the "My Clients" section of your dashboard. You won't be able to transfer documents if your client is in the "Invitations" section.
Once you've signed in to your client's account, go to the File Vault (Menu > File Vault.)
You may click and drag documents to drop within the dotted lines, or upload local files from the green button in the upper right hand corner. When your documents are uploaded, you may adjust the file name, category, or add a description.
Your client will receive a notification that you have accessed their File Vault.