For 64-bit Systems Only:
Open terminal by pressing Ctrl+Alt+T then run the following commands,
wget http://download.cdn.viber.com/cdn/desktop/Linux/Viber.zip
unzip Viber.zip
cd Viber
./Viber.sh
In .deb method
wget http://download.cdn.viber.com/cdn/desktop/Linux/viber.deb
sudo apt-get install gdebi
sudo gdebi viber.deb
Or
wget -O viber64-NoobsLab.com.deb http://download.cdn.viber.com/cdn/desktop/Linux/viber.deb
sudo dpkg -i viber64-NoobsLab.com.deb
Direct download of .deb
Download from Viber.com
cd /viber_download_location/
sudo dpkg -i viber.deb
For 32-bit Systems:
You can use sudo dpkg --force-architecture -i viber.deb command, but use of wine is advisable.