Many “free download” sites hijack your browser, install adware, or use your computer as a proxy for botnets.
Ensure you have every file in the split sequence. If the download had 12 parts (.001 through .012), you need all 12. Missing a single part makes the archive unopenable.
Cracked versions cannot connect to Ubisoft’s servers. You’ll never play the fantastic 4-player co-op mode or access the online map editor.