If you simply search for "download Microsoft Office 2016 installer" on Google or Bing, you will see dozens of third-party websites offering direct .exe or .iso files. Here is why:

use random websites offering “free Office 2016 installer download.” The few extra minutes you save are not worth the risk of ransomware or identity theft.

You cannot download and install Office 2016 without a valid product key unless you are only reinstalling on the same device where it was already activated. If you have lost your key: