Once you have your .zip file downloaded:

Before we proceed, a hard truth: Most websites offering a free "Euro Truck Simulator 2 Zip file" are scams or contain malware.

If you see a file size that is less than 5GB, delete it immediately (the real game is ~7GB+). To stay safe, you have two options:

Windows 10 has native zip support; you do not need WinRAR or 7-Zip, though they offer more options.