Yes! Once you open the itinerary in the Trip Plans app, you have the option to toggle offline mode at the top. Like this:
Once you tap on the toggle this will download the itinerary for you and you will see once it has finished downloaded because it will look like this:
Please note that not all images and maps may download instantly. So it's best practice to tap into each day and view all attachments, places, etc. while online because the app will download and save this information that you view so that you can view it again offline.