If your question refer to this library, you could define the style attribute "page-orientation" on your body tag.
You could have also search the forum for existing question and find this thread: https://html2openxml.codeplex.com/discussions/434042.
I contains also some OpenXml code from StackOverflow (or just look at the code source).
<bodystyle="page-orientation:landscape"> ... </body>
I contains also some OpenXml code from StackOverflow (or just look at the code source).
