Each Bible study you generate creates its own PDF file, but it would be good to be able to combine several studies into a single PDF, treating the combined file as a book. For example, each study in a series of topical studies on election might explore a different group or person that God calls elect. These studies would naturally combine as a single book titled "Who are the Elect?" with each individual study being a chapter in the book. The same idea would apply to a series of expository studies. The title of the book would be the overall scripture passage studied, say, 2 Timothy, and it would combine each individual study of each sub-passage. If the topic or overall passage exposited is large then the studies could be organzed into a hierarchy which would then generate a book with several parts and several chapters within each part.
A book would be more than just related PDF files stuck together into a single file, but would add what publishers call front matter and back matter, for example a title page, a table of contents, and an index. The PDF would also have bookmarks and other links to make electronic reading easier.