Mendeley allows you to highlight and add notation to your full text content through the Mendeley Desktop App.
Open a full text article in the Mendeley Desktop App. From here you can add highlights and notes which you can share with other Mendeley users.
Notes added to a document will appear on the main dashboard under the annotations tab of an entry. Clicking on the note will bring you in to the document at the point where the note resides.
The notebook feature allows you to create notes spanning multiple documents.
To do so, first open the PDF of the document.
Next highlight the section of text you are interested in importing into your notebook.
After highlighting the text click on the highlighted area again. You will be presented with the option to import the highlighted text into your notebook.
If this is your first entry in your notebook, you will be asked to create a page, otherwise you will be prompted to select a page to add your note to.
You can also create notes in your notebook independent of notes in your individual documents.