functions – How to load domPDF to WordPress?

I want to use this library:

https://github.com/dompdf/dompdf

Inside my WordPress in order to generate a PDF, but don’t know how to load in order to use the:

$dompdf = new Dompdf();

Questions:

  1. I there a way to load inside the functions.php?
  2. Where i have to put the directory withouth Composer?

Thanks