Okay, but you should know that releasing a model as freeware doesn't mean it's public domain. Maybe the original author has released it for free only under some terms of usage.
That's what is called a license, like CreativeCommons for example. Please read
Creative Commons licenses - Wikipedia, the free encyclopedia for more information.
In this case, if the original author is unknown, and if the license under which the model was released is also unknown, you can do what you want, until the author shows up and proves he's the author and asks you something.