There are numerous ways to add items to Zotero, which we have not mentioned in this guide.
Please see this Zotero Documentation page for more information.
Adding from a finished bibliography:
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.