Older articles do not have DOIs, so you can attach the pdf but you'll need to input the info yourself.
To import a PDF file:
Open the library into which you want to import the references.
From the File menu, select Import > File to open the Import File dialog.
Click the Choose button to locate and open the file you want to import from your hard drive.
Select the needed PDF file, and then click Open.
Select PDF from the Import Option list.
Select an option from the Duplicates list:
Import All: Imports all references, including duplicates.
Discard Duplicates: Imports all references except duplicates.
Import into Duplicates Library: Duplicate references are imported into a library called File-Dupl.enl, where ”File" is the name of the library into which you are importing.
By default, a reference is considered a duplicate when the Author, Year, Title, and Reference Type match a reference already in the library. See Duplicate Preferences if you would like to change the duplicates criteria.
Select a Text Translation option. The default value is No Translation.
This option allows you to specify the text character encoding of the file you import, so that extended characters transfer correctly. You should verify the setting with your database provider. Some U.S. databases that contain extended characters require ANSI-Latin1 or UTF8. MARC format files may require ANSEL translation. You might be able to use No Translation for text files that do not contain any extended characters.
Click Import to import the file to your library.
Note: See Importing PDF Folders to Create New Records (in EndNote Help) for instructions on how to import multiple PDF files stored within a single folder.
Note: When performing the import function, if there are two DOIs found in the first two pages of a PDF document, EndNote will put both DOIs in the DOI field. The PDF is then attached and the file name is put in the Title field.