Answer
A direct hyperlink straight to the individual attachment itself isn't readily achievable through a plain CSV/Output Channel export. A practical compromise: create a custom field that displays a link back to the actual invoice/parent record (rather than the specific attachment), and export that via Output Channels alongside the invoice data — the user can then follow that link to the record and access its attachments from there. If native links directly to attached documents are a hard requirement, OData feeds (available from Apps10/Aurena, consumable from Excel) may offer more flexibility here, though native attachment-level linking through that path isn't confirmed either.