If you have known the detailed origins of documents (title/ ISSN/ volume...etc), you can quickly obtain the journals or databases NLC purchased,and then get the Link of the full text of the document. If there is no full-text , you can obtain other related clues, such as NLC catalog.
DOI, i.e. Digital Object Identifier, is the unique marker to mark the digital resources.
PMID ， i.e. PubMed Unique Identifier ， is used to mark each article in the journals of biomedicine science.