The easiest way to do this is to download the .deb file from the wkhtmltopdf project github. For example, downloading wkhtmltopdf 0.12.5 for Ubuntu 20.04 on amd64:
$ wget https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.5/wkhtmltox_0.12.5-1.focal_amd64.deb
Then you can install the .deb file with apt:
$ sudo apt install ./wkhtmltox_0.12.5-1.focal_amd64.deb