Go to cadence.com and log in to the portal. You need a free account. (Note: While the viewer is free, access to the download portal may require registration as a customer. For strictly viewing, some third-party repositories exist, but for safety and legality, use official channels.)
Search the knowledge base or product downloads for . The exact file name is typically: cadence allegro free physical viewer 16.6 download
Cadence viewers are generally backward compatible (a 17.2 viewer can usually open a 16.6 file), but they are not forward compatible. If you try to open a 16.6 file with an ancient 15.x viewer, it won't work. Installing the 16.6 viewer ensures you can open designs created in that specific era. Go to cadence
Use the "Display > Measure" command to check: some third-party repositories exist