1) Import the reference for the PDF you'd like to attach if it is not already in your EndNote library
2) Download the PDF if you have not already done so. I temporarily save a copy to my desktop but it can be put anywhere you prefer.
3) In EndNote, open the reference which you would like to attach the PDF to and click on the paperclip button
4) Locate where you have saved the PDF and open it
5) It is now attached to the reference and can be searched, highlighted, e-mailed, and so on
6) Alternatively, you can drag and drop the PDF from your desktop into the open record in EndNote and it will attach itself