Simple Apps Script to automate the "Export As PDF" action of a Google Docs document

ExportPdf.gs

function main() {
  const document = DocumentApp.getActiveDocument();
  const pdfFileId = ScriptProperties.getProperty('pdfFileId');

  if (!docWasUpdatedAfterPdf(document, pdfFileId)) {
    return
  }

  updatePdfFileContent(document.getAs('application/pdf'), pdfFileId);
}

function updatePdfFileContent(fileContent, fileId) {
  Drive.Files.update({}, fileId, fileContent);
}

function docWasUpdatedAfterPdf(document, pdfFileId) {
  pdfLastUpdated = DriveApp.getFileById(pdfFileId)
    .getLastUpdated();
  docLastUpdated = DriveApp.getFileById(document.getId())
    .getLastUpdated();

  return pdfLastUpdated < docLastUpdated;
}

The script should be attached to a Google Docs document to export it in a existing PDF file.
The initial content of the PDF file is not relevant and it can also be empty.
The PDF file id (used by Google Drive to identify the document) should be inserted in a property for the script with the name pdfFileId.