Auto-Tune 5 is a legacy version of the industry-standard pitch correction software by Antares Audio Technologies. Because it has been discontinued for years, finding a legitimate "Zip Download" usually refers to the official legacy installers
Conclusion
Q: I only want the DLL file – is that illegal?
A: Yes – distributing or downloading the standalone VST without a license is copyright infringement. Autotune 5 Vst Zip Download