Sources, uploads and the "corpus" model of research
A notebook can ingest hundreds of files at once — and every answer will cite the slice it used.
A notebook is only as good as the sources inside it. The tool currently accepts PDFs, Google Docs, Google Slides, plain-text files, Markdown, arbitrary web URLs, YouTube videos with transcripts, and raw pasted text. During 2024 and 2025 Google began a staged rollout of audio-file ingestion and spreadsheet support; both are live for Plus-tier subscribers and in partial rollout on the free tier.
The free tier currently caps a notebook at fifty sources and roughly 500,000 words of combined text. The Plus tier raises that to three hundred sources per notebook and a much higher word-count ceiling. Sources can be reordered, renamed, hidden from the model temporarily, or removed entirely without losing your notes. A single source can be cited by every note you save inside the same notebook, which is what makes the tool function like a research carrel rather than a chat window.
Citations in the tool are not decoration. When a reader clicks a footnote, the exact passage — often a single paragraph or even a sentence — is highlighted in the source pane. That behaviour is the feature most quoted by reviewers from the Stanford AI portal and it is the reason many enterprise teams have moved their intake reading into a shared notebook rather than a shared folder.
Walk through NotebookLM sources & uploads →