Kopernio can import RIS files and PDFs to your Mendeley library.
With the Mendeley Web Importer extension, you can save PDFs from your Locker directly to your Mendeley library.
Within the Mendeley Desktop application, you can set up your preferred/default PDF download location as a 'Watched Folder'.
For example, if this location on your computer is User > Downloads > kopernio, you can select the kopernio folder in Mendeley > Settings/Preferences > Watched Folders.
Every PDF that you download in the watched folder will be added to your Mendeley library automatically.
Kopernio Premium users can sync their Locker to Dropbox.