David Kimura PRO said almost 5 years ago on Creating PDF files with WickedPDF :
If you've set up the application with the `vendor/assests/stylesheets/pdf.css` and have this set to complile separately, you can download the CSS from https://getbootstrap.com/docs/3.3/customize/?id=08c164bd0450fe50e9bf and paste it into the `pdf.css`. This is what I'm using. You can probably also do the same for bootstrap 4.1, but the important bits would be the things selected where it is mainly the media styles, grid, typography, etc.