1) For pdfs, Adobe provides the option to secure individual files with a password. The individual downloading the file will need to have the file password in order to view its content. The individually secured pdf can then be loaded the site in Download Manager Pro the same way unsecured files are loaded.
2) For a whole group of files, you may want to work with IT to place these files in a SharePoint site and require users to login to the Georgia State campus network (not WordPress) using their CampusID and Password to access the files. These files can still be linked and referenced on the public website by placing the url to the file in SharePoint in the url for the file in Download Manager Pro in WordPress instead of uploading the actual file to WordPress. When a user clicks on the link to the file, if they are not already logged in to the Georgia Stare campus network, they will be asked to enter their CampusID and Password before being redirected to the file.
If you need assistance with Download Manager Pro, submit a ticket at gsu.uservoice.com or email us at firstname.lastname@example.org.
If you need assistance with securing pdfs or SharePoint, contact the Georgia State IT help desk at email@example.com.