Rotate PDF Pages Online Rotate specific pages.

The tool uses client-side JavaScript with React to handle PDF document processing. It relies on the `pdf-lib` library to load and manipulate PDF files, specifically utilizing the `PDFDocument` class to access and modify page properties. When a user selects a PDF file, it is loaded into memory as an array buffer, which is then passed to the `PDFDocument.load()` method to create a document object. This object provides methods for getting the page count, accessing individual pages, and saving the modified document.

The `parsePages` function is used to parse the user's input string, which specifies the pages to be rotated. It splits the input string into parts based on commas and hyphens, then uses a set data structure to store unique page indices. The function also performs validation checks to ensure that the specified page numbers are within the valid range of 1 to the total page count. Once the page indices are determined, the tool uses the `degrees` function from `pdf-lib` to calculate the new rotation angle for each page.

The tool's UI is built using React components, including `FileDropzone` and `InputWithHelp`, which provide a responsive and interactive interface for users to select files and input page numbers. The `useCallback` hook is used to memoize functions, such as `handleFile` and `process`, to prevent unnecessary re-renders and improve performance. When the user clicks the "Rotate Pages" button, the `process` function is called, which saves the modified PDF document as a blob and creates an object URL using the `URL.createObjectURL()` method. The resulting URL is then stored in the component's state and used to generate a download link for the rotated PDF file.

Error handling is also implemented using try-catch blocks to catch any exceptions that may occur during PDF processing, such as invalid file formats or corrupted documents. If an error occurs, an error message is displayed to the user, providing feedback on what went wrong. The tool's use of browser APIs, such as the `URL` API and the `Blob` interface, enables it to perform all operations client-side, without requiring server-side uploads or processing. This ensures that sensitive documents remain private and secure throughout the rotation process.