How to use the pdf-lib.PDFDocumentWriter.saveToBytes function in pdf-lib

// JavaScript runtime (e.g. Node, the browser, or React Native).

// Here we add the page's to the document. This step is what will cause the
// pages to actually be rendered when the document is opened. Our previous calls
// to `pdfDoc.createPage` only **created** the pages, they did not add them to
// the document.
pdfDoc.insertPage(0, page1).addPage(page3);

// Now we'll convert the `pdfDoc` to a `Uint8Array` containing the bytes of a
// PDF document. This step serializes the document. You can still make changes
// to the document after this step, but you'll have to call `saveToBytes` again
// after doing so.
//
// The `Uint8Array` returned here can be used in a number of ways. What you do
// with it largely depends on the JavaScript environment you're running in.
const pdfBytes = PDFDocumentWriter.saveToBytes(pdfDoc);

/* ========================== 7. Write PDF to File ========================== */
// This step is platform dependent. Since this is a Node script, we can just
// save the `pdfBytes` to the file system, where it can be opened with a PDF
// reader.
const filePath = `${__dirname}/modified.pdf`;
fs.writeFileSync(filePath, pdfBytes);
console.log(`PDF file written to: ${filePath}`);

// JavaScript runtime (e.g. Node, the browser, or React Native).

// Here we add the page's to the document. This step is what will cause the
// pages to actually be rendered when the document is opened. Our previous calls
// to `pdfDoc.createPage` only **created** the pages, they did not add them to
// the document.
pdfDoc.addPage(page1).addPage(page2);

// Now we'll convert the `pdfDoc` to a `Uint8Array` containing the bytes of a
// PDF document. This step serializes the document. You can still make changes
// to the document after this step, but you'll have to call `saveToBytes` again
// after doing so.
//
// The `Uint8Array` returned here can be used in a number of ways. What you do
// with it largely depends on the JavaScript environment you're running in.
const pdfBytes = PDFDocumentWriter.saveToBytes(pdfDoc);

/* ========================== 6. Write PDF to File ========================== */
// This step is platform dependent. Since this is a Node script, we can just
// save the `pdfBytes` to the file system, where it can be opened with a PDF
// reader.
const filePath = `${__dirname}/new.pdf`;
fs.writeFileSync(filePath, pdfBytes);
console.log(`PDF file written to: ${filePath}`);