hi arcticwolf,
i have performed a test to preview a pdf files in outlook 2016, and it works.
also, i can find pdf preview handler in the trust center.
could you check if the outlook 2016 client is up to date?
above is the version of my test outlook 2016 client.
if the issue persists with the latest version, it is suggested you post the issue on our outlook client forum since our forum focuses on office 365 for business online services.
thanks for your understanding.
franky