Merge PDFs by Alternating Pages Interleave pages from two PDF documents alternately.

The tool works under the hood by utilizing the `pdf-lib` library to load and manipulate PDF documents. When a user drops two PDF files into the interface, it triggers the `loadFile` function which uses `arrayBuffer` to read the file contents and then loads the PDF document using `PDFDocument.load`. This process ignores any encryption on the PDFs, allowing it to accurately count the number of pages in each file. The page counts are stored in the component's state as `pagesA` and `pagesB`.

When the user initiates the interleaving process, the tool uses `Promise.all` to load both PDF documents into memory simultaneously, ensuring that both files are fully loaded before proceeding. It then creates a new merged document using `PDFDocument.create`. The tool iterates through the pages of both documents using the maximum page count of either document as the loop limit. For each index, it copies the page from document A (if available) and then document B (if available) into the new document using `copyPages`, which faithfully reproduces the original pages without modifying their contents.

The resulting merged document is saved to a blob using `merged.save`, and its URL is created with `URL.createObjectURL`. This allows the user to download the interleaved PDF directly from the browser. Throughout this process, the tool handles potential errors by catching exceptions and displaying error messages to the user. It also cleans up object URLs when the component unmounts to prevent memory leaks, using `useEffect` to revoke the object URL when it is no longer needed.

The use of `useState`, `useCallback`, and `useRef` hooks in the tool enables efficient management of its state and side effects, ensuring that the interface remains responsive even during resource-intensive operations like PDF merging. By leveraging these React features and the capabilities of `pdf-lib`, the tool provides a reliable and user-friendly solution for interleaving PDF pages from separate documents.