How to make website visitors to have upload function

I need to create upload function for site visitors. They will have browser button, when pressed it will pop up a window, where they can browse their computer and select which file to upload to a page like a link to a file (It will only be PDF, not that it matters). All that would be available for visitor after they enter a password.

So, my problem is how to make that work.. any ideas guys?
Thanks in advance.