The PDF books that Penny Press creates comply with the PDF/A standard.
By default, Adobe Acrobat Reader DC opens a PDF/A standard document as read-only.
When you open the file, Reader shows a message just below the menu:
“This file claims compliance with the PDF/A standard and has been opened read-only to prevent modification.”

If you wish to use Adobe Acrobat Reader DC to add sticky notes and make other changes, you need to follow this procedure:

Click the “Enable Editing” button
You will see the message:
“Editing a PDF will cause the PDF to no longer comply with the PDF/A. Are you sure you wish to continue?”
Click OK.
You can then edit and save the file.