I had the same issue before, but I can't remember the project to find the fix, you should try [Bootstrap Printing] (https://blog.donnierayjones.com/2014/10/fix-bootstrap3-printing/) or other similar fixes, but from what I remember it's an issue with the grid system breaking when you are trying to print.
Good luck, and please report back so I can bookmark it this time.
I just discovered yesterday that Dompdf doesn't load css files. There is a way to do it properly but what I ended up doing was including the css file inline in the html.