Introduction
If you have ever tried to build a document-heavy AI app — a RAG interface, a contract review tool, an internal spreadsheet inspector — you already know the ugly truth: the viewer layer eats weeks. PDF.js is a rabbit hole, DOCX rendering is a compromise, XLSX is worse, and none of it knows anything about your embeddings, citations, or agent state. Extend UI is a React component kit that tries to close that gap, and it's fully open source under MIT.
I spent time integrating it into a document Q&A prototype. This is a builder-to-builder read on what it actually does, where it holds up, and where the tradeoffs show.
Key Features
The scope is narrow on purpose. Extend UI is not a generic component library — it is a document UI toolkit aimed at AI and agent workflows.
- Multi-format viewers. PDF, DOCX, XLSX, and CSV React components in one kit. That is a rare combination; most open-source options force you to stitch together three libraries with three rendering models.
- Bounding box citation overlays. This is the headline feature. You pass coordinates from your model output and the viewer draws the citation on top of the source. For RAG apps, this alone can justify the install.
- File upload and file system browser. The unsexy plumbing you always end up writing — provided, styled, and consistent with the rest of the kit.
- E-signing component. Not full DocuSign, but enough to wire signature capture into a document workflow without a third-party embed.
- Schema builder and document splits view. Useful when you are training extraction models or letting users define fields against a source doc.
- Drop-in ready for agent pipelines. The components accept structured inputs that map cleanly to LLM outputs, so you are not writing translation layers.
Pricing Breakdown
There is one plan: free. The entire kit is MIT-licensed and self-hosted.
| Plan | Price | What you get |
|---|---|---|
| Open Source | $0 | All components, MIT license, self-hosted, full GitHub source access |
No hosted tier, no paid tier, no enterprise SKU. The commercial angle for Extend AI (the company) is their extraction platform — the UI kit is the on-ramp. That is worth knowing, because it shapes the roadmap.
Pros & Cons
Pros
- Fully open source, MIT license, ~1.4k GitHub stars at launch — signal that people are actually using it.
- Covers all the major document formats in one kit with a consistent API. You do not learn four rendering models.
- Citation and bounding-box support is genuinely rare in open source and directly useful for RAG and agent apps.
- Ready for both agent-driven and user-facing flows without custom wiring or adapter layers.
Cons
- React-only. No Vue, Svelte, Angular, or web-component variants. If your stack is not React, it is a non-starter.
- Maintained by Extend AI. The roadmap will reasonably favor what their extraction product needs, not necessarily what your app needs.
- Narrow scope by design — if you are not building around documents, there is nothing here for you.
- No hosted or managed option. You own the integration, the updates, and any custom rendering edge cases.
Who Is It For
Extend UI is a strong fit if you check most of these boxes:
- You are building a React app.
- Documents (PDFs, spreadsheets, contracts) are central to the product.
- You are shipping AI features — RAG, extraction, summarization, agents — that need to point users at the source.
- You want to stay in the open-source lane rather than embed a hosted viewer.
It is a bad fit if you are on Vue or Angular, if your app only occasionally shows a document, or if you need a fully managed viewer service with SLAs.
Verdict
Extend UI fills a real gap. Document viewers that natively understand AI citations and agent context are not something you find in every UI kit — and rolling your own bounding-box overlay on top of PDF.js is a project, not a component. For teams building RAG interfaces, document automation, or internal tools around PDFs and spreadsheets in React, Extend UI is a meaningful head start and the price is right.
The honest caveats: the React-only constraint is a hard filter, and the fact that it is stewarded by a company whose main product is document extraction means you should evaluate whether the roadmap will keep aligning with yours. For most teams in the target audience, those are acceptable tradeoffs.
Recommendation: If you are on React and shipping document-centric AI features, install it this week and see how far it gets you. The cost of trying is a few hours; the payoff is potentially weeks of viewer work you never have to do.
Rating: 7.2/10