If your BBCode editor supports HTML attributes, then all you need to add to your .pdf link is the `target="_blank"` attribute. Like so:
Code:
[a href="#" target="_blank"]Link text[/a]
If not, you could write a script that appends this attribute to all links that end in .pdf:
jQuery:
Code:
$('a[href$=".pdf"]').attr('target', '_blank');
Vanilla:
Code:
let link = document.querySelector('a[href$=".pdf"]');
link.setAttribute('target', '_blank');
However, you might consider the points made about opening links in a new tab from this CSS Tricks article:
https://css-tricks.com/use-target_blank/