const PDFMerger = require('pdf-merger-js');

var merger = new PDFMerger();

(async () => {
  merger.add('pdf1.pdf');  //merge all pages. parameter is the path to file and filename.
  merger.add('pdf2.pdf', [2]); // merge only page 2
  merger.add('pdf2.pdf', [1, 3]); // merge the pages 1 and 3
  merger.add('pdf2.pdf', '4, 7, 8'); // merge the pages 4, 7 and 8
  merger.add('pdf3.pdf', '1 to 2'); //merge pages 1 to 2
  merger.add('pdf3.pdf', '3-4'); //merge pages 3 to 4

  await merger.save('merged.pdf'); //save under given name and reset the internal document
})();