WKHTMLTOPDF
Download the deb file for the wkhtmltopdf library
sudo wget https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.1/wkhtmltox-0.12.1_linux-trusty-amd64.deb
Download the Libpng12 library
wget http://mirrors.kernel.org/ubuntu/pool/main/libp/libpng/libpng12-0_1.2.54-1ubuntu1_amd64.deb
Install the Libpng12 library
sudo dpkg -i libpng12-0_1.2.54-1ubuntu1_amd64.deb
Install now the wkhtmltopdf deb file
sudo dpkg -i wkhtmltox-0.12.1_linux-trusty-amd64.deb
Copy the binary files to the bin locations:
sudo cp /usr/local/bin/wkhtmltopdf /usr/bin
sudo cp /usr/local/bin/wkhtmltoimage /usr/bin
Restart Odoo Server, (You may need to refresh Odoo on your Browser to start downloading PDF reports)