Claude Sonnet is the strongest fit for summarizing long PDF reports and contracts because it accepts large document uploads directly, handles long context windows well, and produces clear, structured summaries of dense legal and business text. You can paste or upload a PDF and ask for an executive summary, key clauses, or a section-by-section breakdown. NotebookLM is a compelling free alternative that lets you upload multiple PDFs and query them interactively, making it ideal when you need to cross-reference several documents at once. Gemini Pro edges ahead when you want a free-tier option with a very large context window for especially lengthy files. ChatGPT works well if you are already in the OpenAI ecosystem and want GPT-4o's file-upload and summarization features.