Finding a legitimate "free download" for AutoCAD 2013 is complicated because Autodesk has officially retired it.

Here’s what you need to know about getting and running AutoCAD 2013 today. Can You Still Download AutoCAD 2013 Legally?

Autodesk typically allows subscribers to download the current version and up to three previous versions directly from their Autodesk Account.

For versions released 4 to 5 years ago, you must use the Autodesk Assistant (the chatbot on their support page) to request a download link.