Because Calibre is already free, any site offering a "working serial key" for it is a major red flag:
Calibre is a powerful, free e-book management suite used to organize libraries, convert file formats, and sync with e-readers like Kindle and Kobo. It is developed by volunteers and relies on donations rather than a subscription model. Why You Should Avoid "Crack" Sites Because Calibre is already free, any site offering
"Crack" files are often used as vehicles for viruses, ransomware, or spyware. Because Calibre is already free
Downloading from unverified third-party sites exposes your computer to security threats. Calibre 6.12.0 Overview convert file formats
Shifted to the Qt 6 framework, which dropped support for Windows 8. Download for Linux - Calibre