You can add files to Mendeley by:
Export references directly from other Elsevier products such as Science Direct or Scopus to Mendeley by:
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Mendeley Institutional Edition users can now try out "Mendeley Suggest" which suggests new papers based on your library.
Try it today by clicking "Mendeley Suggest" which is located on the left panel below "Literature Search".
Install the Mendeley Web Importer -- Install the Web Importer into your favorite web browser to import references and documents from academic databases.
Save articles to your Mendeley Library as you browse.
You can have Mendeley 'watch' certain folders on your computer, then as PDFs are added to these folders they get imported into Mendeley automatically. Choose File - Watch Folder in Mendeley Desktop.