Mendeley allows you to annotate and highlight PDFs that are uploaded into your Mendeley library. Once a PDF is uploaded, click on the PDF icon that appears next to the reference.
This will open a new tab within Mendeley with the PDF displayed. You can then use the icons in the Annotations toolbar at the top of the page to highlight, leave notes, etc. To navigate back to your library, click the 'My Library' tab underneath the Annotation toolbar.
For additional help and more information about annotating PDFs in Mendeley, click here.