Doug has given you good advice.
On the other hand, there is no simple answer to your question.
Many different factors may came into play. Just to mention one or two: what kind of story you've told, what kind of relationship you have with your subject, what role your subject has in the documentary, the way you've told their story, etc., etc.
In some cases I'd say it would be best NOT to show it to them before your film comes out (goes to a festival, airs on tv, is released in the theaters, on DVD, etc.), in others, there wouldn't be any valid reason not to show it to them privately.
Unless you give us some additional details about your film and your relationship with your main character, it's almost impossible to say what might be best or more appropriate in this particular case.