Articles are not automatically attached to references when exported into EndNote - even if the article is available to download with the reference it will not export with the reference into EndNote
PDF articles either need to the found via the 'Find fulltext' function, OR downloaded and saved either as a file on your PC, to your desktop or on a USB, and then attached to the appropriate reference in your EndNote Library.
For assistance in attaching files to your EndNote Library, please see the above video, or for further assistance with EndNote please complete the online help form