$ sudo apt-get update
$ sudo apt-get install pepperflashplugin-nonfree 

pepperflashplugin-nonfree is installed in your system.

Make ensure the pepperflashplugin-nonfree package were installed using the commands given below,

$ sudo dpkg-query -l | grep pepperflashplugin-nonfree *

You will get with pepperflashplugin-nonfree package name, version, architecture and description in a table

pepperflashplugin-nonfree package basic information: