Want to update Samsung Galaxy On7 to Nougat
Yes, that links seems safe to me. It tells you to download the official firmware for your device from the official Samsung page or SamMobile, which is a trustworthy website. The problem is, you won't find the Nougat firmware there either, because it's not released yet. The only thing you can do is wait until Samsung officially publishes Android 7 for your device. 

By the way, flashing any kind of Android firmware is never illegal, since Android is open source (only exception is when paid content is included for free). It can only void your warranty.