1. Open a PDF in Mendeley's PDF viewer by double clicking on it in the central pane.
2. Add highlights and notes by clicking on the Highlight or Note buttons on the menu.

You can share your annotations with your co-workers:

1. Open a PDF that is part of a Group
2. Make your annotations
3. Sync these annotations to Mendeley Web by going back to "My Library" and clicking on "Sync Library"