7zip is open source so from what I know and I checked some tech fora, if you got it from a reliable place like CNET or the 7zip site itself it should be OK. But from someone/place else it's possible that it could have been messed with and something attached. .
That is true. Even with the best, safest applications, if you're not careful in choosing the websites from which you download them, you might end up downloading add-ons that you don't want. You also need to be careful when running the downloader/installer that you uncheck the check box for every add-on that you don't want, for many installers will try to "sell" you something you don't want. The best choice for downloading 7-Zip is from the 7-Zip site itself. Outside of that, CNET is a good alternative.