The option to download only the free version no longer exists. If you download Malwarebytes (or simply click on an 'Update' button when you see a message telling you that there's a new version available) you
automatically get a free trial of the full version (which reverts to the free version after a month if you don't pay up).
However I've found that having the full version can slow my computers to a crawl while it works in the background, so I've now ditched Malwarebytes Anti-Malware altogether.
AdwCleaner used to be from a rival company but Malwarebytes bought it and continues to update it. (Therefore it should be just as good as Malwarebytes Anti-Malware at finding and zapping anything dodgy). It's got a more basic interface than Malwarebytes Anti-Malware but it's completely free and doesn't have a 'premium' version to tempt you into buying. So that's what I now use (and recommend to others):
https://www.malwarebytes.com/adwcleaner/