In Mendeley Reference Manager, click the PDF icon or doubleclick the entry in your Library to open a document in the PDF Viewer.
In the PDF Viewer you can:
See also Mendeley's Guide to Reading and Annotating Your Documents.
Mendeley Notebook can be used to keep track of notes on a particular topic or project that may pertain to multiple references in your Mendeley library (notes about a specific reference can go in the General Notes field under Annotations). You can have multiple Notebook pages saved under different titles. You can add your own free text (notes) to your Mendeley Notebook, and can also choose highlighted text from multiple papers to show up in your notebook. This is a way to collect quotes or facts that you might want to refer back to in your own writing.