The current workflow to add a pdf to a reference in cases where PP was unable to automatically download the pdf (it happens) is to download the pdf from the publisher site. Then select the reference in PP, click add PDF, choose a file, then choose file (again- as opposed to folder), which finally opens the file/folder window.
It would be great to have a keyboard shortcut that would immediately open the file/folder window to add a pdf file for a selected reference.
Thank you for the suggestion. At the moment, you can select the paper in your library, which will give you the option to attach a file whenever you have a PDF opened in your browser: