By default, EndNote shows attached PDF using the somewhat limited built-in PDF viewer. By right-clicking the PDF tab in the reference panel or pressing CTRL+ALT+P, it is possible to open the PDF in the default PDF application on the computer. However, some people might have more than one PDF application and the default one might not be the one needed when opening a PDF from EndNote.
For example, I have Acrobat as my default day to day PDF application. However, when I need to read and annotate a PDF, I prefer to use PDF Annotator which allows me to use my stylus on my Surface Pro and hand write notes on my PDF just like on paper.
It would be nice if we could specify in EndNote settings the preferred PDF application to use. That way, when opening a PDF from EndNote, I would always use PDF Annotator because my intentions are to actively read and annotate the document.