Cross-Platform PDF Export App

I’m creating a lightweight mobile component that will run on both iOS and Android and turn users’ existing files into polished PDFs. The workflow is straightforward: the user picks one or more files already stored on their device, the module merges them into a single document, lets the user add quick annotations (text notes, highlights, simple drawings), and then saves or shares the finished PDF.

What I need from you is a clean, well-documented codebase that:

• Accepts multiple local files, orders them, and merges them into one PDF
• Provides an in-app annotation layer—basic text, free-draw, and highlight tools are enough
• Exports the final document to the file system and the standard share sheet on both platforms

You are free to leverage native SDKs or cross-platform frameworks such as Flutter or React Native, as long as the final build process produces stable binaries for the latest public versions of iOS and Android. Third-party libraries are fine too, provided their licenses allow commercial use and the code remains fully buildable in a fresh environment.

Acceptance criteria
1. Source code builds without errors in a clean clone on macOS and Windows.
2. A simple demo screen shows the merge-annotate-export flow end-to-end.
3. Generated PDFs open correctly in Apple Books and Google Drive viewers, with all annotations embedded.

Once the functionality is solid I’ll handle UI polish, so focus on the underlying exporter logic and clear separation of platform-specific code. Let’s keep the initial milestone focused on delivering the working exporter module; we can discuss extra features later.