Exporting Specific Pages From a PDF as a New PDF Using Google Apps Script

async function myFunction() {
  // Retrieve PDF data.
  const fileId = "###"; // Please set a file ID of your a PDF file or a file ID of Google Docs files (Document, Spreadsheet, Slide).
  const pageNumbers = [2, 4]; // In this sample, 2 and 4 pages are exported as a PDF.

  const blob = DriveApp.getFileById(fileId).getBlob();

  // Merge PDFs.
  const cdnjs = "https://cdn.jsdelivr.net/npm/pdf-lib/dist/pdf-lib.min.js";
  eval(UrlFetchApp.fetch(cdnjs).getContentText()); // Load pdf-lib
  const setTimeout = function (f, t) {
    Utilities.sleep(t);
    return f();
  };
  const pdfDoc = await PDFLib.PDFDocument.create();
  const pdfData = await PDFLib.PDFDocument.load(
    new Uint8Array(blob.getBytes())
  );
  const pages = await pdfDoc.copyPages(
    pdfData,
    [...Array(pdfData.getPageCount())].map((_, i) => i)
  );
  pages.forEach((page, i) => {
    if (pageNumbers.includes(i + 1)) {
      pdfDoc.addPage(page);
    }
  });
  const bytes = await pdfDoc.save();

  // Create a PDF file.
  DriveApp.createFile(
    Utilities.newBlob([...new Int8Array(bytes)], MimeType.PDF, "sample.pdf")
  );
}