I think the best way to get wkhtmltopdf working on Ubuntu 14.04(or other version) is get the lastest version from website project from http://wkhtmltopdf.org/downloads.html
Get the right version for your system Like Ubuntu 64 bits or 32 bits, or your for system you didn't mentioned.
use "sudo dpkg" to install the package
sudo dpkg -i wkhtmltox-0.12.1_linux-trusty-amd64.deb # if you downloaded for 64 bits
maybe you got errors because of libc6, if happens try to fix it with:
sudo apt-get -f install
After that, you have the updated version of wkhtmltopdf from package wkhtmltox which removes wkhtmltopdf package if installed.
You can check if wkhtmltopdf is working with:
wkhtmltopdf http://www.odoo.com test-odoo.pdf
Check out pdf file is odoo website.
After that, restart Odoo server and try again.