Should I add tag free on different product pages on my store?

Is it good idea to add tags like free on product pages in my store? The idea is to rank when someone searches for free product like free book on this topic.

Short answer - no. It's not a realistic thinking that you will rank for "free iphone" for example and you will only make your user experience worse that way. Sorry to "shit' on your idea - that's just my opinion.