What material is used for the floor panels in the main cabin for Boeing 747-400?
This illustration is earlier than a -400. Later ones might differ. There are two types: the normal one has aluminum skins bonded to a PVC core, for galleys/hold it is titanium skin bonded to a PVC core with greater strength/corrosion resistance.



747 cutaway view



Later a honeycomb sandwich was used, apparently two cross plied glass-epoxy prepreg skins bonded to phenolic coated nomex honeycomb core using an epoxy film adjesive. From:


  Handbook of Adhesives and Sealants: volume 1, 2005