While rarely pursued for individual users, using a cracked license is software piracy. The Digital Millennium Copyright Act (DMCA) makes it illegal to circumvent software protection. More realistically, the "crack" website may sue you or threaten legal action (though this is rare; they usually just steal your data).
If you encounter issues with your license key, try the following: Fileviewpro License Key
Some "license key" packs install browser extensions that you cannot remove. Your search engine changes to a fake Yahoo version, and every click opens two pop-up ads. This is low-grade malware, but it is incredibly annoying to remove. While rarely pursued for individual users, using a