Returns a short-lived (15 minute) pre-signed URL for the invoice's source document. Fetch the returned url to download the file.

By default the original uploaded document is served. Pass stamped=true to receive the confirmation-stamped copy instead; the stamped copy is generated on demand the first time it is requested. If a stamped copy cannot be produced, the response falls back to the original document — check the variant field to see which version the URL points to.

Returns 404 if the invoice does not exist, is outside your company's data, or has no source document (for example, records migrated from an external system).