At the top of the summary panel, there is an Attach file button.
Click that to browse for and attach a PDF document to the current selected reference.
The PDF file will now be visible at the top of the summary panel when you select the reference.
After attaching a PDF, click on a different reference in the list to indicate to EndNote that you have finished attaching. The next dialogue box will prompt you to save the attachment to the reference.
The PDF document can be annotated with highlighting, strikethrough, underlining, and "sticky notes". The PDF text will also become available for searching in EndNote.
To read and annotate a PDF document, click the PDF icon in the summary panel and select Open in the dropdown menu. The PDF will open within EndNote in a new window.
For more editing functionality select Open with Adobe Acrobat DC.
To annotate the PDF:
When you close the PDF you will be prompted to save your changes.