When websites offer these for "free," they are usually providing one of the following:
The "activation certificate" is typically part of a manual (offline) activation process for paid users who cannot connect to the internet. Trial and Activation - Nitro PDF
While an individual user is rarely sued, companies that use cracked software face massive fines. Nitro Software actively monitors for mass distribution of fake certificates. If you use a cracked version at work, your company’s IT department can be tracked, resulting in fines of up to $150,000 per instance.