First run the VizAlerts test scripts, it’s either test #23 or test #24 that has portrait and landscape views in a merged PDF, if this works then we know the problem is something with the configuration that you’re using. Assuming that works, then what you’ll need to make sure you have set up is in Tableau Desktop set the page formatting for each of the dashboards or views to default to portrait or landscape orientation and then republish the workbook(s).
Did this answer your question, Arnold? If so, could you please mark the answer as correct? Thanks!