Hi @yazdzik,
I currently have kernel 4.4 in UM 16.04!.
wolfman@fred:~$ uname -r
4.4.0-18-generic
wolfman@fred:~$
For Skype:
For Viber:
http://viber-for-linux.en.uptodown.com/ubuntu
Flash can be problematic but runs okay mostly, use another browser like Google Chrome which works fine!:
For PDF readers (as @LinuxHippie pointed out) :