Although you can’t edit existing text, you can add new text. In Preview, you can strike through, underline, highlight existing text, add comments, and add shapes. So to open a PDF with Preview on a Mac, just double-click the document you want to view. Preview is the default option when opening PDFs on a Mac. MacOS, the Mac operating system, includes Preview, a free PDF Mac app that lets users open, view, and comment on PDF files.