If you have a valid product key, here is how to get the exclusive offline ISO legally:

In the pantheon of Microsoft Office releases, 2013 stands as a peculiar, transitional artifact. It followed the universally-adopted Office 2010 and preceded the cloud-obsessed, subscription-driven Office 2016/365 era. But for a specific subset of users—enthusiasts, offline purists, IT administrators, and software archivists—the Microsoft Office 2013 ISO remains a uniquely compelling way to experience this suite. This review examines Office 2013 not just as productivity software, but specifically as an ISO-exclusive product: a static, burnable, mountable snapshot of a bygone philosophy of software distribution.

Microsoft maintains a "Backup" site for legacy software. This is the only authorized method to download the digital ISO image without relying on unauthorized third-party repositories.

Procedure:

Note: If you do not have a product key, Microsoft no longer sells Office 2013. You must purchase a subscription to Microsoft 365 or purchase Office 2021.