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 (The Account Owner), your client and your partners on the project. Collaborators, Partners, and Project Managers 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.
We also have a guide on how you or your client can add documents to the project itself here.