There are three main ways to populate your EndNote library with references:
In many library-subscribed databases, you can do a search and export citation information for multiple sources from your results into EndNote. This is a lot faster and more accurate than manual entry.
You can now open any of these references and edit, annotate, or organize them as needed
If you have a PDF file or folder of PDF files, you can import them into your library. When the PDF has a Digital Object Identifier, also known as a DOI, it will generate a reference and bring that PDF into your library. When it does not have a DOI (often the case with older scanned articles), it will only bring the PDF into your library.