DOI and PMID
A DOI is a persistent identifier commonly assigned to scholarly objects such as articles and datasets. A PMID is the numeric identifier used for a PubMed record. They are not interchangeable and a PMID does not necessarily imply a DOI exists.
ISBN and ISSN
ISBNs identify book editions, while ISSNs identify serial publications. Their check digits help detect transcription errors but do not verify the quality, indexing or legitimacy of a publication.
ORCID
An ORCID iD identifies a researcher record and helps distinguish authors with similar names. A valid checksum only confirms the identifier structure; ownership and current profile information should be checked on the official ORCID record.