TapXWorld/ChinaTextbook ? reverse-engineered prompt

I want to build a simple website to host a collection of Chinese educational textbooks. The main purpose is to make these resources freely available to everyone, especially for overseas Chinese families who want their children to learn from them.

The site should present the textbooks in a clear, browsable structure based on the existing folder organization: elementary, middle school, high school, and university levels. Users should be able to navigate these categories to find and download the PDF for each book.

A key feature is handling large files. Some of the bigger textbooks are split into multiple parts (like book.pdf.1, book.pdf.2). I want the website to automatically merge these pieces on the backend. When a user clicks download, they should receive a single, complete PDF file without needing to download and run a separate program to combine the parts themselves. The overall design should be clean, simple, and focused on easy access to the materials.