Zotero imports citation information for the PDFs you add to your collection automatically (known as 'PDF indexing'). When you drag and drop PDFs into Zotero or use the "Store Copy of File" or "Link to File" options from the add new item menu (green plus sign), Zotero will add citation information to the PDFs and rename the associated files (see Zotero's blog about new PDF features for more information). If you prefer, you can disable these automatic functions in the General pane of Zotero preferences (see Zotero's general documentation for instructions).
If you have PDFs without citation information in your library, you can get the information manually. In the middle pane, select the PDFs you want citation information for, right-click on the PDFs, and select "Retrieve Metadata for PDFs."
If Zotero can find a match for the item, it will create a full citation with the available data and attach the PDF:
