Yes, you can attach a document, ie. an airbill or a purchase order that is required for the pickup and/or dropoff process. This can be done during the placing of a delivery request. Unfortunately, you cannot attach the document after the delivery request has been placed or accepted by the driver, yet.
The driver will have access to the document and will be able to present it on his/her mobile device.
The document size is limited to 10 megabytes.