Currently Atticus cannot read PDF files since those are created to be fixed documents. It does not retain the information or formatting necessary for Atticus to import them efficiently.
We always recommend using .docx files for import, so if you don't have that version of the file, you will first need to convert the PDF to a .docx file.
Once you've converted the file, make sure you take a look through this tutorial about our best recommendations for preparing your docx file prior to upload, as it will teach you how to format your content so that Atticus recognizes where to break up your chapters: https://www.atticus.io/how-to-prepare-your-word-document-for-upload/