If you specifically need CS5, here are legal options:
Please keep in mind that I'm providing general guidance and not endorsING or promoting piracy or unauthorized software use. adobe illustrator cs5 portable free link
For complex CS5-era AI files (especially those with gradients, brushes, or transparency), no free tool handles them perfectly. Your best bet is a legitimate Adobe subscription for a single month, then convert everything to SVG or PDF for long-term use. If you specifically need CS5, here are legal
If you're looking for free alternatives to Adobe Illustrator CS5, there are several options available: If you specifically need CS5
If you hate subscriptions, Affinity offers a professional-grade vector suite for a single, affordable fee.