Simpler: just let everyone download an encrypted version of the files. Then only allow your client software that you control to decrypt the downloaded files.
Encrypt the files using AES-GCM. If you trust that your own client software is distributed securely and won't run in hostile environments and be reverse engineered, just ship that AES key with your software. Otherwise it will get complicated fast.
Encrypt the files using AES-GCM. If you trust that your own client software is distributed securely and won't run in hostile environments and be reverse engineered, just ship that AES key with your software. Otherwise it will get complicated fast.