A DOI, or digital object identifier, is a unique and permanent identifier that will take you straight to an article no matter where it’s located on the Internet. The DOI, when available, can be found:
On the first descriptor page of an e-journal article, usually beneath the Abstract:
NOTE: Not all articles have DOIs. Articles published prior to 2000 are less likely to have DOIs. Also, only publishers that meet the obligations of the DOI system and are willing to pay to become a member of that system can assign DOIs.