TrialPad has a limited Whiteboard feature as part of the app. To create a new whiteboard, go to the Key Docs tab, press the Plus button, and tap Add Whiteboard. Use the annotation tools like Pen and Highlight to create your demonstrative. Your whiteboard will be named "Whiteboard" and a thumbnail of your demonstrative will appear beside the Whiteboard in the Key Docs document list. You can rename your whiteboard for quick identification. You can create multiple whiteboards, and all annotations on the whiteboard will automatically be saved in Key Docs tab.
The whiteboard feature is great for basic drawings such as an intersection, however, if you want something more elaborate you can use the Keynote app to create blank slides that you can annotate and then export to TrialPad.