I installed wkhtmltopfd 0.12.1.3 on Ubuntu 18.04 LTS as the following in this post 

https://www.odoo.com/forum/help-1/question/ubuntu-18-04-lts-how-to-install-wkhtmltopdf-0-12-1-134198 

sudo wget https://builds.wkhtmltopdf.org/0.12.1.3/wkhtmltox_0.12.1.3-1~bionic_amd64.deb
sudo dpkg -i wkhtmltox_0.12.1.3-1~bionic_amd64.deb
sudo apt-get install -f
sudo ln -s /usr/local/bin/wkhtmltopdf /usr/bin
sudo ln -s /usr/local/bin/wkhtmltoimage /usr/bi

after doing the above steps, it works for odoo modules 

except for inventory/porduct printing it return the error below

Wkhtmltopdf failed (error code: -8). Message: b''

please advice  

Regards

Ahmed