This page provides the logic for building the correct URL for downloading a file stored on a Nuxeo Document, provided we know the document ID.
You can use the nxbigfile pattern that is executed by a standalone servlet:
http://127.0.0.1:8080/nuxeo/nxbigfile/default/776c8640-7f19-4cf3-b4ff-546ea1d3d496/file:content/rm.pdf
http://127.0.0.1:8080/nuxeo/nxbigfile/default/776c8640-7f19-4cf3-b4ff-546ea1d3d496/blobholder:0/rm.pdf
http://127.0.0.1:8080/nuxeo/nxbigfile/default/776c8640-7f19-4cf3-b4ff-546ea1d3d496/files:files/0/file/SC-DM-DAM.png
http://127.0.0.1:8080/nuxeo/nxbigfile/default/776c8640-7f19-4cf3-b4ff-546ea1d3d496/blobholder:1/SC-DM-DAM.png
Other Documentation About URLs