Click the File #1 button to choose one PDF file and then the File #2 button to choose another (ideally very similar) PDF file, then click the Compare button to perform the comparison, and when that's finished, navigate through the pairs of differing pages using the View combobox or using the Previous and Next buttons. Alternatively, drag two files—either separately or together—and drop them onto DiffPDF's view panels, then click the Compare button.