import { PDFDocument, rgb } from 'pdf-lib';
const pdfDoc = await PDFDocument.load(existingPdfBytes);
const pages = pdfDoc.getPages();
const firstPage = pages[0];
firstPage.drawText('Your text here', {
  x: 100,
  y: 700,
  size: 20,
  color: rgb(0, 0, 1),
});
const pdfBytes = await pdfDoc.save();