You create a microflow with the “Download file” Activity and give the Activity the FileDocument you want to download.
In the Microflow you either already have the File as Parameter or need to retrieve it.
You then call the microflow in a way you’d like to do.
Personally, I would make an entity that uses the system.FileDocument as generalization in my domain model and give it some more self-choosen attribute to retrieve it more easily.