You can change the orientation of the whole document in the following way:
1. Open your file and from the main menu dropdown, set the document to Layout mode.
2. Tap Orientation.
3. Choose Portrait or Landscape orientation.
*For further page customization, tap Page Setup and adjust the changes you need from the bottom sheet.
*Note that it is not possible to have certain pages or sections of your document with a different orientation from the rest of it.