Finding a legitimate "free download" for AutoCAD 2013 is complicated because Autodesk has officially retired it.
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. AutoCAD 2013 Free Download 32 Bit / 64Bit
If you are setting up a legacy workstation for this version, here are the baseline specs you’ll need: System requirements for AutoCAD 2013 - Autodesk Finding a legitimate "free download" for AutoCAD 2013
Here’s what you need to know about getting and running AutoCAD 2013 today. Can You Still Download AutoCAD 2013 Legally? Can You Still Download AutoCAD 2013 Legally
Since AutoCAD 2013 is well over a decade old, it is no longer hosted on official download servers for most users. If you have a perpetual license and the original media (DVD), you can still use it, but Autodesk no longer provides new activation codes for re-installation. AutoCAD 2013 System Requirements
Autodesk typically allows subscribers to download the current version and up to three previous versions directly from their Autodesk Account.