A read-only CADKEY Viewer was historically available to view .prt and .ckd files without owning the full software.
While CADKEY 99 is still available for free download, it's worth noting that it's an older software, and newer versions may be available. The latest version of CADKEY is now part of the Kubotek3D portfolio, and it's available for purchase. cadkey 99 free download upd