If you previously purchased Nitro Pro 8 and lost your key, you can try these steps to retrieve it: Help Me Fix My Activation Problem

Nitro Software no longer supports Nitro Pro 8 (its latest version is Nitro PDF Pro 14+). Even if you activate an old version, it will have security vulnerabilities and compatibility issues with modern Windows 10/11.

Para activar Nitro Pro 8 debes utilizar un número de serie legítimo de 18 dígitos que se adquiere mediante la compra del software