Many people like to highlight or write notes on pdfs while reading papers. ReadCube enables this functionality. You can take notes in two ways. The first is to click on the speech bubble in the .pdf menu of the upper right hand corner of the screen. This opens a My Notes screen that stays open as you scroll through the document.
The second is to highlight text that you are interested in. This opens a dual menu that allows you to either apply highlighter color to the passage that you have marked or open an add note menu.