Be aware that you may be requested to upload a signed document. In this case, your client has attached the document in the respective question that can be downloaded. For more information, see the last section "Adding a signed document".
Adding a regular document
Click the requested document type below "Answer".
Now, you can either choose an existing document that has been uploaded previously or (most likely) you will have to upload a new document. For the latter, add the document file in the designated field. If needed, you can rename the document.
Click "Create Document".
Click "Save".
Adding a signed document
Click the download icon to retrieve the document that needs to be signed.
Click the requested document type below "Answer".
Follow the steps 2 until 4 of section "Adding a regular document".