You can set Mendeley up to see whenever you download a new PDF to a folder. In 'Preferences' or 'Options' go to the 'Watched Folders' tab.
Zotero and EndNote products continue to be supported by their makers. I suggest you investigate Zotero and EndNote and take your queue from Elsivier - abandon Mendeley. They have chosen to not update Mendeley to work properly with Word 2016 for Mac. Elsivier (the company that owns Mendeley) appears to have abandoned support for Mendeley. I added folders within folders to see if the software would scale to the demands of a larger research project. Local, Public, and Private Folders Mendeley reproduced my Zotero folder hierarchy.
Mendeley Desktop for Mac OS X 10.10 or later Mendeley Desktop for generic Linux Mendeley Desktop for Ubuntu/Debian 1. Find Mendeley software downloads at CNET, the most comprehensive source for safe, trusted, and spyware-free downloads on the Web.