This post helped me to resolve my problem : http://www.stormconsultancy.co.uk/blog/development/generating-pdfs-in-rails-with-pdfkit-and-deploying-to-a-server/
I'm reproducing here the step from this post that helped me to install it :
# first, installing dependencies
sudo aptitude install openssl build-essential xorg libssl-dev
# for 64bits OS
wget http://wkhtmltopdf.googlecode.com/files/wkhtmltopdf-0.9.9-static-amd64.tar.bz2
tar xvjf wkhtmltopdf-0.9.9-static-amd64.tar.bz2
mv wkhtmltopdf-amd64 /usr/local/bin/wkhtmltopdf
chmod +x /usr/local/bin/wkhtmltopdf
He also create an initializer to tell to pdfKit where it is, so this method is for wicked PDF and PDF Kit.