If you have a free, downloadable model in your possession, then you could certainly build it and give it away, or give a copy of the files to your brother or friends. You can even build it and sell the completed model. What you can't do is post the model on the internet without permission from the designer. The key is permission.
In some cases it can be argued that it's a gray area though - if the designer dies, or just moves on to other hobbies and forgets about papercraft, and his/her website disappears because they aren't around to pay the bills and keep it running, posting the models somewhere to keep it available may well be exactly what they would want. In a case like that it hasn't been specifically forbidden, but then you don't specifically have permission either, so here in the forums the rule is you don't do it without permission.
With commercial kits it's a little different. If you buy a printed kit, it's expected that you will make exactly one model from that kit. If you scan it into your computer, print it, and make two models, you have infringed on the copyright by duplicating the model. You bought one, but made two.
If the commercial kit comes in a downloadable, or other electronic form (like CD), unless there is a license agreement with the kit that specifically prohibits it, you can make as many as you want, and do whatever you want with the completed models, including sell them. What you can't do, is give away or sell copies of the original model files, since they are copyrighted material.
Hope that makes it clearer.
Steve