Add a PDF to your library by opening Zotero, then drag and drop the PDF into the collection you would like to add it to. If the PDF has been created with Optical Character Recognition, Zotero can retrieve the metadata for the PDF. Either right-click (PC) or control-click (Mac) and select Retrieve Metadata for PDF from the drop down menu.
If you add a PDF that has no metadata attached, Zotero will only show the title of the pdf file - not helpful!
In that case, use the MANUAL add button 
to fill in the title, author, date of the PDF, and then drag the pdf document INTO that entry.