PDFs (and other files) can be dragged from your computer folders to your Zotero library.
- Open Zotero and click on a Zotero folder.
- Drag and drop a PDF into the middle (Title) Zotero panel.
- With Zotero 6, metadata associated with a PDF is automatically retrieved and imported. If it isn't or is incomplete, you can add it manually by right clicking and selecting "Create parent item."