I had the same issue...tried various combinations, didn't think to try <br/>, since that is non-standard syntax. The standard syntax would be: HTML:<br>; XHTML:<br />.
I wish Syncfusion's PDF component were maintained, because oddities like this make it very cumbersome and un-intuitive to use. This is a basic problem that (1) should be well-documented on the relevant pages, specifically:
And (2) the parser should accept the official xhtml tag, which is <br /> and ideally the standard html tag, which is <br>.
Instead the developer is left scratching his head and wasting time unnecessarily on a simple problem. I myself stumbled upon this solution by accident while browsing the forum trying to figure out another problem, after I gave up on finding a solution days ago.