Is there an App that I can use to highlight articles when I read them.
Adobe Acrobat Reader offers comment and highlighting tools, but the tools are only available in PDFs that have commenting enabled. See this support article from Adobe for more information on mark-up tools in Acrobat Reader.
RefWorks has a built-in PDF reader that requires no additional applications. This makes it easy to store articles, read them, and add highlights and comments. You can read and annotate saved articles anywhere with just a web browser.