there may be times when you have a finished paper, with a full bibliography, but you do not have the citations from that bibliography in your zotero account.
if you wish to add citations into zotero, from a bibliography, you have some options as described on this zotero documentation page.
options (and tidiness of the process) depend on:
a) if the bibliography was created using zotero (or another citation manager) - in which case you will likely use reference extractor.
b) if the bibliography was created manually without the help of a citation manager - in which case you will likely use anystyle.
in both instances, you will end up with a file (bibtex for example) which you will then import into your zotero library.
this will bring the citations into zotero - but not the full text .pdfs, which, if needed, you will have to import in other ways.
again, please see the above mentioned zotero documentation page for details.
