Download the packing slip for a shipment. The document is rendered when you call this, from the shipment's current state and the tenant's PDFs & Emails settings, so it is the same document the product's Download button produces rather than a stored copy of one sent earlier. Served in every shipment status, including Cancelled. The file name is composed from the order the shipment fulfils and the shipment's sequence number rather than from the id you request by: `SO1862-2.pdf` for a shipment against sales order 1862, and `PO104-1.pdf` for an outside-processing shipment against purchase order 104. The packing slip is rendered against that order, so a shipment whose order has been deleted, or which is attached to neither, has no document to return and gives a 404.