The free version of Notebooks
  • runs on iPhones only,
  • is based on Notebooks 8,
  • does not receive any relevant upgrades.
The paid version, in contrast,
  • runs on all iOS devices,
  • is actively developed and maintained,
  • receives regular updates and upgrades, and thus offers an improved interface and much more functionality.

You can read more about the functional difference in these articles about the major upgrades Notebooks 10 and Notebooks 11.

What is the Difference Between the Paid and the Free Version?