Viber has released its official client for Ubuntu/Linux. To install it, open your terminal with CTRL+ALT+T and type:

wget https://download.cdn.viber.com/cdn/desktop/Linux/Viber.zip

Then unzip it and cd to that viber directory. Then do as:

unzip Viber.zip
cd Viber
~/Viber/Viber.sh


How to install in .DEB way // Currently only 64-bit available

Open your terminal and type:

wget -O viber.deb https://download.cdn.viber.com/cdn/desktop/Linux/viber.deb  
sudo dpkg -i viber.deb

Source: software installation – How to install Viber in Ubuntu? – Ask Ubuntu