This page describes the ways you can retrieve metadata from the PDFs saved in your Zotero library.
Many researchers new to Zotero already have a large collection of PDFs that they've previously organized with other programs or methods. Zotero has systems in place to make it easier to import these PDFs and retrieve full bibliographic metadata (for searching, citing, indexing, and organizing), making the switch to Zotero easier.
You can follow these steps provided by Zotero to use the "Retrieve Metadata" function for PDFs:
First, drag your existing PDFs into your Zotero library.
You can also use the "Store Copy of File" or "Link to File" options from the Add New Item menu (the green plus sign):