You cannot legally download CS5 for free. However, if you find an old CD-ROM of Adobe Creative Suite 5 on eBay, you can install it. Adobe no longer activates CS5 (the servers are down), but there are official "offline activation" workarounds provided by Adobe for legacy owners.

The Adobe Illustrator CS5 Lite Portable Repack is a repacked version of the original software, which means that it has been re-created to be smaller and more portable. The repack includes:

If you need a lightweight or budget-friendly vector editor, consider these safe, modern options: Alternative Compatibility Professional-grade, free, and open-source vector editing. Windows, macOS, Linux Affinity Designer

Adobe Illustrator CS5 Lite Portable (Repack) – Full Vector Power, Zero Footprint

Using such versions violates Adobe’s End User License Agreement and may constitute software piracy, which is illegal in most jurisdictions.