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;
}

To run this script it is also required to enable the Drive service in the Google Apps Script configurations of the project.