EndNote's Find Full Text tool will automatically search the web for PDF files of the references in your library. EndNote searches a few free resources, but it can also search AU's subscription databases for PDFs.
To get the EndNote find full text feature to work with AU library resources you will need to adjust some settings:
The Find Full Text preferences menu should look like this:


Note: The find full text feature will not be able to find and attach PDFs to all of your references. If you want PDFs attached to all of your references you will need to manually attach many of them.