The Digital object identifier (DOI) is a unique identifier attributed to every published electronic document such as research articles. The worldwide DOI database is known as CrossRef. The DOI is the best way to link papers in a web-page or in a curriculum.
For instance, the paper by Jeffrey Lin entitled ￼A negative answer to a problem of Aldous on determination of exchangeable sequences published in 2016 in Electronic Journal of Probability has DOI 10.1214/16-EJP4403 (the part before the slash indicates the publisher). This means that it is available via http://dx.doi.org/10.1214/16-EJP4403. This URL redirects to the right page on the publisher website. This redirection is handled by the CrossRef system. The CrossRef database is updated by all the publishers around the world. By this way, if a publisher changes his website, he will update the CrossRef database and DOI users have nothing to do. This resembles in a way the Domain name system (DNS) used for the Internet protocol.
Recently, in collaboration with Project Euclid, I have managed to add the DOI to the first page of the PDF file of all “old” ECP and EJP papers published in volumes 1 to 16. This was done on my Debian distribution using essentially the PDFtk utility.