Well, I think the problem is that you need to have a blob object, this blob object will be linked to the file that you want to download, then just get the blob.
Check this link, the have one example about how to upload and download a file. https://doc.nuxeo.com/display/NXDOC/Using+Nuxeo+Automation+Client --- Mailing list: [email protected] Forum: http://forum.nuxeo.org/f/1/
