how download linux os for pc free ?

There are many different "distributions" of Linux. They all offer something a little different. Here are three distros that are popular among the community. You will be able to download a distribution of your choice at any of these sights.

Keep in mind there are far more than the three I just posted, but its hard to know which one you'll end up going with. Check out some of the distro specific threads on this sight to get a better idea of what you're looking for! :)
When you say "free" that hints that you expect like windows (genuine) there will be a charge. Well #Linux and a lot of programs are GNU licence free ;that means you can download with the lead /core team support for free with no looking over your shoulder. Most would appreciate though some financial support

eg For slackware this gives you an insight

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