First select notebook root like this:

Then press Saving options button in the Notebook properties panel. A Notebook saving options dialog will appear.

Check the text after Current notebook status, it should be 'Not compressed', if so - press Compress current notebook button. After a short time period the window will close and notebook compressing process finishes.

You can check the successfully notebook compression in Notebook properties panel - after the Flags text a 'Compressed' text should be. If so - notebook compressing has been successfully completed.