/**
 * ### Description
 * Add page numbers to PDF.
 *
 * @param {Object} blob PDF blob.
 * @param {Object} pageFormat Format of page number.
 * @returns {Blob} Updated PDF blob.
 */
async function addPageNumbers_(blob, pageFormat) {
  if (blob.getContentType() != MimeType.PDF) {
    throw new Error("Blob is not PDF.");
  }

  // Load pdf-lib
  const cdnjs = "https://cdn.jsdelivr.net/npm/pdf-lib/dist/pdf-lib.min.js";
  eval(
    UrlFetchApp.fetch(cdnjs)
      .getContentText()
      .replace(
        /setTimeout\(.*?,.*?(\d*?)\)/g,
        "Utilities.sleep($1);return t();"
      )
  );

  const data = new Uint8Array(blob.getBytes());
  const pdfData = await PDFLib.PDFDocument.load(data);
  const pdfDoc = await PDFLib.PDFDocument.create();
  (await pdfDoc.copyPages(pdfData, pdfData.getPageIndices())).forEach(
    (page, i) => {
      const { width } = page.getSize();
      const obj = { center: width / 2, left: 20, right: width - 20 };
      const pageFormatObj = { ...pageFormat };
      pageFormatObj.x = obj[pageFormat.x];
      page.drawText(`${i + 1}`, pageFormatObj);
      pdfDoc.addPage(page);
    }
  );
  const bytes = await pdfDoc.save();
  return Utilities.newBlob(
    [...new Int8Array(bytes)],
    MimeType.PDF,
    `new_${blob.getName()}`
  );
}

// Please run this function.
function sample1() {
  const fileId = "###"; // Please set the file ID of the PDF file.
  const pdfBlob = DriveApp.getFileById(fileId).getBlob(); // Of course, you can directly give the PDF blob.

  const pageFormat = { size: 10, x: "center", y: 10 };
  addPageNumbers_(pdfBlob, pageFormat).then((newBlob) =>
    DriveApp.createFile(newBlob)
  );
}

// This function is a simple demonstration script.
function sample2() {
  // Create a sample Google Document.
  const tempDoc = DocumentApp.create("tempDoc");
  const body = tempDoc.getBody();
  for (let p = 0; p < 5; p++) {
    body.appendParagraph(`sample text ${p + 1}`).appendPageBreak();
  }
  tempDoc.saveAndClose();
  const pdfBlob = tempDoc.getBlob();

  // Add page numbers.
  const pageFormat = { size: 10, x: "center", y: 10 };
  addPageNumbers_(pdfBlob, pageFormat).then((newBlob) =>
    DriveApp.createFile(newBlob)
  );
}