Is it permissible to download non-free antivirus software from websites that provide it for free?

It is not permissible to use copied programs. There is no difference between the original version, if copies are made from it, and the copied version. Owning the original version does not legitimize infringing upon the rights of the author or manufacturer. However, if the manufacturing company itself chose to make the program free, then it is permissible to benefit from it.