Yes, it can be applied to every document, a book would be more expensive, because it has more text and thus more input tokens. The pdf needs to be converted to text, because the API only accepts text, some equations which can be written using Unicode are directly put into the network and it can understand. Other equations are currently skipped. So far I have spent almost 100$ in tokens to summarize the papers, so there need to be some paid features in the near future or a reduction in the amount of papers.