You can download Portofino 5.3.1 binaries, API documentation, and source packages from SourceForge: https://sourceforge.net/projects/portofino/files/manydesigns-portofino.
Library dependencies to build Portofino applications are available on Maven Central and on npm for the Angular UI. Also, a Docker image for the standalone Upstairs application to manage a Portofino instance is available on Docker Hub.
Finally, on the GitHub wiki you can find:
a Getting Started page for users that are new to Portofino
a growing collection of video tutorials
a document detailing all the changes from Portofino 4 to Portofino 5.0 and some migration instructions from Portofino 4.