pepperflashplugin-nonfree downloads the Chrome for Linux Debian package (.deb) from Google and extracts the Flash PPAPI plugin (
libpepflashplayer.so) to make it available to other browsers system-wide.
pepperflashplugin-nonfree is broken since Google changed the method of distribution of the Flash plugin with Chrome 54 for Linux;
libpepflashplayer.so is not directly bundled in the Debian package anymore.
A bug report is available on Launchpad.
Since the Flash plugin is often affected by critical vulnerabilities and
pepperflashplugin-nonfree currenlty fails to update the plugin,
pepperflashplugin-nonfree should not be used anymore and either
adobe-flashplugin from Canonical Partner or no Flash plugin at all should be used.
If you use Flash you can check the version of your Flash plugin on this support page on the Adobe website.