First of all Portal is available on PC and Mac. This is again same the previous post Free full version games, You need to install Steam from Valve Corporation to get this free.
1. You need to create an account in Steam create it here
2. Go here after the account creation
2. Click install Game
3. If you have not installed Steam Click "No, I do not have Steam" download and install (1.5MB)
4. After install click "Yes, I have Steam" (steam will download and install it)
The total download is nearly 5GB
More free games here