Steps:

1. At first install xvfb server:

   sudo apt-get install xvfb

2. Get needed version of wkhtmltopdf from http://wkhtmltopdf.org/downloads.html.For Ubuntu 14.04 64-bit:

   wget http://download.gna.org/wkhtmltopdf/0.12/0.12.2.1/wkhtmltox-0.12.2.1_linux-trusty-amd64.deb

3. Install wkhtmltopdf:

   sudo dpkg -i wkhtmltox-0.12.2.1_linux-trusty-amd64.deb

   Install dependency (if needed):

   sudo apt-get -f install