ChatPDF (Chat with Your PDFs)
A service where you upload a PDF and then ask questions about its contents. Answers include citations, so you can see which part of the PDF they came from. Besides PDFs it accepts Word, PowerPoint, Markdown, and text files.
How to try it
Drag a file onto the upload area on the front page or paste a link, then type a question to start the chat. Create a folder and add several files to work with multiple documents in one conversation. You can start without an account, but uploaded files go to the operator's servers, and the free plan is stated as two documents per day.