"A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically."

So you can access a journal article by going to http://dx.doi.org/DIO-GOES-HERE. doi.org doesn't host files, just resolves them to the current and correct location. For example the DOI number for the first paper in TFA is 10.1007/11427186_42 so it can be accessed at http://dx.doi.org/10.1007/11427186_42

You know the DOI you know where you can find it.