PDF documents are automatically retrieved when you use the Zotero Connector, this is the best way to add items to your library. However, if you have PDF files already saved on your computer and you want to add them to your library, Zotero Desktop can create a bibliographic record from the PDF.
Simply drag the file into Zotero library central pane, this is also the easiest way to attach a PDF file of an article to its corresponding entry in your library:
You can also attach files to an existing item in this way:
If you want to extract the text that you have highlighted on a PDF file, you will need to download a Zotero plugin called ZotFile.
You can extract annotations you've made on a PDF file, in order to do so you will have to download Zotfile first, if you haven't downloaded already.
To extract annotations: