It seems as though you might be mixing up PMID (PubMed ID) and PMCID (PubMed Central ID) as they are in fact different database identifiers. PubMed is basically a giant citation database, whereas PMC actually hosts articles from thousands of journals and also inludes the massive data enrichment ecosystem, of which PMCID is one.

What is the difference between a PMCID and a PMID?

A Pubmed Central reference number (PMCID) is a unique identifier for full-text article in PMC. A PubMed reference number (PMID) is a unique identifier for a citation record in PubMed. A PMCID is used as evidence of compliance with the NIH Public Access Policy.

https://pmc.ncbi.nlm.nih.gov/about/faq/

If that isn't it then I'm not quite sure, but as a side note I can let you know that bulk downloading of PMC content outside their dedicated FTP service for the Open Access subset violates PMC ToC due to copyright infrigement and they can (and do) block IPs due to detected bulk downloading through the regular web service.