Open the document editor by tapping the grid button at the top right
Make sure you are in the edit mode. To switch to the edit mode tap the edit button next to the grid button
Tap the plus button at top left and configure the page in the visible dialog
Tap the "Add" button to add the page
Save by tapping the checkmark button at top right and choose a save option
Note: Any changes that are not confirmed will be lost when leaving the document editor!