Because PDF is an evolving format that was initially just used to provide displaying of printed documents, but now including PDF-UA 2 tagging that provides appropriate tagging for distinguishing headings, table titles, list structure etc, often one PDF file can be used appropriately by screen readers while a similar looking file can not. First question to consider:
Were you prompted to use auto tagging when viewing the document? If so, then this means the PDF creator did not create a fully accessible version of the document. One can often find discussions about this on accessibility support discussions.
I currently don't know how to determine if the tagging of the PDF meets a specific PDF-UA standard.
HTH,

Jonathan