DOIs and PMIDs are unique identifiers to help locate a specific article. For example, this PubMed citation has both a DOI and a PMID:
Improving health literacy in community populations: a review of progress. Nutbeam D, McGill B, Premkumar P.Health Promot Int. 2018 Oct 1;33(5):901-911. doi: 10.1093/heapro/dax015. PMID: 28369557