Include Custom HTML block on certain downloads?

Howdy!

What’d be the best way of including a Custom HTML block on multiple pages but If I edit the “main” it updates all pages? Currently if something changes I manually edit every page with the new information.

However, now I’m at 28 pages it’s a bit of a drag and I’m sure there is a faster way! Obviously I can’t just make a customblock.php file and just do <?php include 'customblock.php';?> like here as WordPress doesn’t allow PHP.

Of course, there is WPCode Plugin (which I use for other things) but this will include it on every page I only want it on certain download items/listings under specific topic/category.

Thanks!