You can take notes on documents in Mendeley, as well as highlight portions of them.
1. Highlight a resource title. Click on Notes, in the right hand column of Mendeley Desktop, between Details and Contents.
2. Add as many notes as you'd like about the resource.
3. Back in Details, scroll down to open the PDF.
4. From here, you can add highlights to portions of the article by clicking and dragging. The left icon highlights text, the middle one highlights text and adds a note, and the right one copies the text to the clipboard.
5. Here is my newly highlighted text.