The external end user wants to download the API document from API portal. Does API portal support document download ?
If an API owner added a WSDL file for the API on the Portal, then developers can download the API's documentation by going to the Portal's Documentation page, selecting the API, and then clicking Download Documentation.
I don't believe there is currently a way to do this with a WADL (or I haven't had success in doing so). However, if you publish a SOAP API from the API Portal (Portal Published API) and add a WSDL and a path for a documentation page, you should see a button for "Download Documents" on the documentation page associated with that API.