Stored Files or Linked Files, such as PDFs can be attached to your citations. Highlight the citation in the middle pane, choose the paperclip icon and select Attach Stored Copy of File or Attach Link to File. Alternatively drag and drop the file by resizing your browser window with Zotero in it so you can open a second window and open it to the folder with your saved files. Drag the file from your folder on top of a citation to add and attach it (or drag between items to create a standalone item).
What’s the difference between a Stored File and a Linked File?
Stored file attachments use Zotero.org server space! Files stored with the Zotero server are limited to 300MB of free storage space and sync between multiple computers and group libraries. Additional storage space is available for a fee. The Attach Link to File option creates a path to a file stored on your hard drive. Linked files do not sync.