Navigate to the project you want to add files to by selecting Projects on the left navigation panel and then selecting the corresponding project.

Once in the project, select Documents from the top tool bar.

This will open the documents section and you can see any files currently uploaded. On the right side of the page, select New Documents and then Upload Project Document to choose a file to add.

After uploading the desired document, select it via the checkbox, navigate to the Portal Access dropdown, and make sure to select "enable portal access" so your investors can see the document.