Don't think there is any plugin that does as desired, however, with some scripting of your own, one could accomplish. It's means making a query of db mdl_files table, finding the contenthash and file name of the .pdf cert. Then using the contenthash ... finding the file in moodledata/filedir/ (uses contenthash value for location and filename in the sea of files there in.
Then copying those out to a certs directory outside of Moodle.
Download from there.
If you don't have tons of courses, one could make a full user backup of the courses - that should include certs. Then use similar process without database ... extracting the files directory and the files.xml file. The files.xml is a map of files in files directory, and like server, files.xml has a contenthash value + a filename xml tag. Those could be used to find the certs in the extracted file directory and from there one could copy the contenthash named .pdf cert to the same name to a manually created certs directory by humanly recognizable file names taken from files.xml file.
'SoS', Ken