Even if the ISO is genuine and malware-free, you might download an old build that is vulnerable to known exploits like EternalBlue or PrintNightmare. When you install it, your fresh system is compromised within minutes of going online.
An index of Windows 10 ISO files is useful for several reasons:
Before installing on bare metal, mount the ISO in a virtual machine with no network access. Look for unexpected behavior: pop-ups, changed browser homepages, or system slowdowns.