When an invoice is pushed to a client account it will create a corresponding purchase invoice within their account. Unfortunately at this time, it doesn’t display the attached files on the original sales invoice. We are looking to include these early in 2017.
In the mean time you can ask the supplier to include the following token on their email that is sent with the invoice:
@AttachedFileList@.
This will then output that file list on the email itself. There’s a little more about this feature here.