Custom fields accept HTML. But themes are written in Liquid. And what displays on screen is your computer’s/browser’s rendering of HTML, while a PDF is the result of a PDF generator. So you’ve got four different things going on.
Manager is not a WYSIWYG program. Expecting it to be just leads to frustration. You are better off to use simple <br> tags.
If that doesn’t solve your problem, remember this is not a coding forum. Hire a local programmer who can write Liquid if you don’t have the skills.