Once you have created a Mendeley account, you can also download the desktop Mendeley Reference Manager app. Mendeley allows you to organize your library in either the web version or the desktop version, and then sync the two from your desktop. Navigation of your library is similar within both versions of Mendeley. On both the desktop app and online version, you have access to your library and your groups. The Notebook feature allows you to annotate and highlight within your PDFs and other documents.
Mendeley's layout has three panels representing different levels of information about your references:
Double clicking on an item with a corresponding PDF will open it in Mendeley's PDF reader where you can save notes and annotations.