Perplexity is an answer engine that reads the web for you and writes a sourced summary, with footnote-style citations under every claim. Free tier covers everyday lookups; Pro ($20/mo) unlocks unlimited Pro Search, Deep Research, and the Comet browser agent. The Sonar and Search APIs let developers wire the same real-time web Q&A into their own products.

ChatPDF is an AI assistant for reading and querying PDFs, Word docs, PowerPoint files, YouTube videos, and websites, with answers anchored to clickable source citations. Built for students, researchers, and professionals working through long documents.

Scite is an AI research platform that uses Smart Citations to classify every reference as supporting, contrasting, or mentioning a claim, with the surrounding sentence shown in context. Aimed at academic researchers, students, and analysts working across scholarly articles, patents, clinical trials, grants, and preprints.

NotebookLM is Google's AI research notebook that grounds every answer in sources the user uploads (PDFs, Google Docs, slides, web URLs, YouTube videos, audio files), with citations linking back to exact passages. Aimed at students, researchers, and knowledge workers synthesizing material across documents.