By attaching PDFs to references in EndNote, you can highlight, annotate, and sync articles to your EndNote Online account.
To manually attach a PDF -
Select the reference you would like to an add an attachment
Right-Click (cmnd-click for Mac) -OR- choose Edit from the toolbar -OR- click the paperclip icon in the references toolbar
Highlight File Attaments
Select Attach File
Navigate to the location of the PDF and select it, and click Open