One of the strengths of Mendeley is its Read and Annotate features. You can highlight text and create sticky notes within the document, and you can create annotations for each reference.You can also keep notes across references using Mendeley Notebook.
After opening a PDF, use the tools at the top of the page to highlight and create sticky notes within the text.
Click on the Annotate tab in the right pane to add annotations for the individual reference you are viewing.
Create a Notebook to take notes on a topic across references.
Access your Notebooks from the main Mendeley library page, on the left.