You can attach PDFs or other types of files to EndNote references. The simplest way to attach a file to a reference in your library is to do it manually. There are 3 basic ways of attaching a file manually.
You can import PDFs into your EndNote library. EndNote X4 and later can extract the reference information using its Digital Object Identifier (DOI). If the DOI is not present the PDF will be imported, but you will need to do manually enter the reference information into EndNote.