Client can easily upload files to your projects right from the Client Portal. All they need to do is navigate to a project, then click the "+" symbol next to the Documents section.
Note: File uploads are visible only to you, your client and your partners on the project. Collaborators do not currently have access to project files.
From here, clicking the link "Upload File" will show a modal they can use to select and upload files to be shared.
This will add the file to your project so you and your client can access it in the future.