Convert PDF to a Different Version Online Convert PDF version.

Under the hood, it utilizes client-side JavaScript with React to load and process PDF files entirely within the browser. The `useState` hook is used to store state variables such as the uploaded file, page count, current version, target version, processing status, result, and error message. The `useCallback` hook memoizes functions like `handleFile`, `process`, `download`, and `reset` to prevent unnecessary re-renders. When a PDF file is dropped or selected, the `handleFile` function reads the file using the `arrayBuffer` method and extracts the current version from the PDF header using a regular expression. The `pdf-lib` library is then used to load the PDF document and retrieve its page count.

The tool uses the `TextDecoder` API to decode the PDF header bytes and extract the version information, while the `TextEncoder` API is used to encode the new version header bytes when converting the PDF version. When the user selects a target version and clicks the "Convert Version" button, the `process` function overwrites the version header bytes in the loaded PDF document with the new version bytes and saves the modified document using the `save` method of `pdf-lib`. The resulting PDF file is then created as a blob and its URL is generated using the `URL.createObjectURL` method. If an error occurs during processing, it catches the error and displays an error message to the user.

The responsive UI is built using React components like `FileDropzone` and `SelectWithHelp`, which provide a user-friendly interface for uploading files and selecting target versions. The UI also features buttons for converting, downloading, and resetting the tool, with disabled states handled using the `disabled` attribute. When the conversion is complete, it generates a download link for the resulting PDF file, allowing users to save the converted file to their local system. By leveraging these technologies and APIs, it provides a powerful and flexible solution for converting PDF versions online.