You can take notes on documents in Mendeley, as well as highlight portions of them.

1. Highlight a resource title. Click on Annotations, in the right hand column of Mendeley Desktop, between Info and Notebook.

2. Add as many notes as you'd like about the resource.

3. Back in the Info tab, scroll down to open the PDF.

4. From here, you can add highlights to portions of the article by clicking and dragging. 

 5. You can also make comments on sections of the article.