Family Tree Notebooks pages are PDF pages that do not have boxes for you to add type. They are not "fillable" because I want you to be able to customize the page as much as you can so your finished product shares your family tree exactly the way you want it to.
I have a post about how I use Goodnotes to fill in the pages, another post for Adobe Acrobat, and one for the other PDF editors that people in our community are using. I promise, even if you're intimidated by learning a new program, you can definitely do this! It's also a lot of fun to have free reign over how you want to use the pages and I'm excited to see what you come up with.