When you important a citation into Mendeley, you can also import a PDF version of the article. By reading the article in Mendeley, you can easily take notes, highlight important text, and search for keywords.
Open the Mendeley application, and right click the article you want to read. Select "Open File."
To make a general note:
To make a note in a specific location:
There are two options for selecting what you want to highlight. For most articles, you can use either option. However, some PDF files are scans of the original text document. They are effectively a picture of the text. To highlight these articles, you will need to use the Highlight Rectangle option.
After you've select which option you want to use, click and drag to highlight the text. You can then right click to add a note, change the color of the highlight, or copy the text.