First, enable PDF indexing on the Search tab of Zotero's preferences. Zotero will download and install a small plugin.
Next, just drag your PDF files into the Zotero pane.
Right-click the PDFs and choose "Retrieve Metadata for PDFs." Zotero will retrieve their citation data from Google Scholar and turn them into citeable items with PDF attachments.
If Zotero can't find a match on Google Scholar, 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.