To import the PDF on Layer 1 and automatically add a annotation/scratch layer we will select these options:
'Import into Layer 1' (Imports the PDF onto the base layer)
'Work in Layer 2' (Adds a annotation layer automatically and sets it as the active layer)
Step 4: (OPTIONAL) Adding a Layer to an Existing Notebook
Use the Layer icon to dropdown the layers manager.
Select 'Add Layer'
Automatically the notebook will add Layer 2, and enable editing on this new layer. You can go back and erase items on Layer 1 by opening the layer manager and choosing 'Work on layer 1'