Your information might be outdated -- In the latest version of Zotero, when you add a PDF to Zotero, it automatically retrieves the metadata via DOI lookup.
(I don't think it would work with papers too old to have DOIs but haven't checked lately.)
The metadata retrieval pairs with a DOI or ISBN [1]. Articles without such handles might not work, nor will articles which are not OCR'ed (obviously) but this can be easily fixed with OCR scanning software.
The metadata retrieval process is a very handy feature, especially paired with Zotero's web browser plugin.
Zotero also keeps a repository of all the citation styles, which are curated and administered by the Zotero team and stored in a Github repo [2].
(I don't think it would work with papers too old to have DOIs but haven't checked lately.)