Extend UI: Open-source UI component kit for PDF, DOCX, and XLSX document viewers.

What it does

Extend UI is a collection of 14 open-source React components for building document viewing and handling features. The kit includes viewers for PDF, DOCX, XLSX, and CSV files. Beyond viewing, it offers file upload, bounding box citations, e-signature functionality, and a schema builder. Components are designed to integrate into user-facing applications, AI agents, or internal tools.

Who it is for

Developers and product teams building applications that need document handling capabilities. This includes document management platforms, AI-powered tools that process files, internal administrative systems, and any product requiring native document viewing without external iframe dependencies. Teams looking to customize viewer behavior to match their product design will find the component approach useful.

Pricing

The site does not show prices.

How it stands out

Extend UI is MIT licensed and fully open-source, meaning developers can inspect, modify, and fork the code. The creators built it because existing document viewer libraries and component kits lacked either the full feature set or the polish needed for production use. The component-based design allows customization at a granular level rather than using embedded viewers. Support for multiple document formats in a unified component library reduces fragmentation when handling different file types.

What a founder should check

First, examine incumbent document viewers and component libraries. Verify what features existing solutions like PDF.js, Syncfusion, or cloud-based viewers like Figstack already offer and how they compare on customization depth and code maintainability. Second, assess switching costs for teams currently using commercial document solutions. Understand what integration effort is required and whether the open-source approach truly reduces total cost of ownership versus paid alternatives with support and maintenance guarantees. Third, evaluate the moat. With an MIT-licensed project, anyone can fork the codebase. Determine how Extend AI plans to monetize—whether through complementary hosted services, premium components, enterprise support, or cloud integration tools—since the core library itself is freely copyable.

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