Another good option is Unriddle. It builds an AI assistant on top of any document, giving you immediate answers, summarizing complex information and helping you draft documents. It can handle large PDF files and works in more than 90 languages, so it's particularly good for researchers and scientists.
If you prefer a chat interface, AskYourPDF and PDFGPT.IO offer similar abilities. AskYourPDF uses ChatGPT to let you have a conversational interface with documents, while PDFGPT.IO gives you immediate answers to PDF questions, which can be useful for improving projects and learning. Both offer flexible tools to boost productivity.