Cadence updates their portal frequently. If 16.6 is not immediately visible under the main product list, look for a tab labeled "Archived" or "Previous Releases" . If it is completely removed, it means Cadence has ended official support for that version's standalone installer. In this case, you may have to download a newer viewer (17.2, 17.4, or OrCAD Viewer), as newer viewers are usually backward compatible with 16.6 files.
utility to update the 16.6 file to a newer format compatible with the current free viewer. Third-Party Archives : Community forums occasionally mention unofficial archives cadence allegro free physical viewer 16.6 download
It allows engineers, technicians, and managers to: Cadence updates their portal frequently