Because official links are gone, many users rely on third-party archives. against known official values to ensure the ISO has not been tampered with. End of support for Windows 10, Windows 8.1, and Windows 7

Do not use Windows 8.1 on any PC connected to the internet for banking, shopping, or sensitive work. Without patches, it’s vulnerable to over 800 known exploits.

You have downloaded the ISO. Now you need to make it bootable.