It's easy to add PDFs to your Zotero library and automatically import their citation info.

Just drag your PDF files into the Zotero pane.

 Zotero will attempt to retrieve citation data from Unpaywall and Google Scholar and turn the "naked" PDF into a full citation parent record with with PDF attachment.

If Zotero can't find a match on your PDF, don't worry -- you can still save the citation from another catalog or article database, then drag the PDF onto the citation to make it an attachment.

You may have stand-alone PDF in your Zotero library that lacks any citation info. 

You can try right-clicking the PDF and choose "Retrieve metadata for PDF."  Zotero will try to look up the PDF information in sources such as Crossref, UnPaywall, or Google Scholar. 

If the metadata is found, Zotero will create a citation and attach the PDF to it as a child record. Success will depend on the amount of metadata encoded in the original PDF.  If the PDF is merely a scan or image of text, you will have to create or download a citation and then attach the child PDF manually.