PK files (packed raster) contain font bitmaps. The output from Metafont includes a generic font (GF) file and the utility gftopk produces the PK file from that. There are a lot of PK files, as one is needed for each font, that is each magnification (size) of each design (point) size for each weight for each family. Further, since the PK files for one printer do not necessarily work well for another, the whole set needs to be duplicated for each printer type at a site. As a result, they are often held in an elaborate directory structure, or in 'font library files', to regularise access.