Flexible and rigid ducts can be run across an attic floor. It is important to insulate it well to avoid ice dams that can lead to heavy water damage to an attic. Flexible duct work is easier for homeowners that are installing duct work themselves.
Running duct work through an attic makes a lot of sense if more ducts need to be added to an existing system or simply to more easily run a new duct work system. Flexible ducts are especially useful in easily supplying heat to top story rooms. Duct work would go across the attic floor and through floor joists to service the rooms below.