Businesses love ISOs because they can store them on a USB drive or network server. If a hard drive crashes, you can restore Office quickly without hunting for old download links.
Websites like SoftwareKeep, StackSocial (with caution), or Amazon sometimes sell legitimate, unused Office 2013 product keys that include a download link to an ISO hosted on their CDN (Content Delivery Network). Ensure the seller is a Microsoft Partner.
Solution: You cannot mix 32-bit and 64-bit Office. Uninstall any existing Office (even 365) using the official Microsoft Office uninstall tool, then try again.