The DOI for each article has to be added to the pdf version of the full text article each time. This is usually done by the layout editor, during the layout editing phase. Add it to the header or footer area.

Examples:

http://www.scielo.org.za/pdf/sajs/v110n11-12/12.pdf

http://www.ajlmonline.org/index.php/ajlm/article/viewFile/194/182