WKHtmlToPdf is a very common tool to generate PDF files from HTML code. In order to install version 0.12.5 (suitable for Odoo 14) on Ubuntu 20.04, just follow the below steps :

Download the package

wget https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.5/wkhtmltox_0.12.5-1.bionic_amd64.deb

Install the package

sudo dpkg -i wkhtmltox_0.12.5-1.bionic_amd64.deb

Fix dependencies

sudo apt install -f

That's all folks, now you're done !