ブラウザのクライアントサイドのみでバイナリを編集して結果を保存する

const FROM_CHAR_CODES = Array.from('/Registry (Adobe) /Ordering (Japan1) /Supplement ').map((c) => c.charCodeAt(0));
const TO_CHAR_CODES   = Array.from('/Registry(Adobe) /Ordering(Identity) /Supplement ').map((c) => c.charCodeAt(0));

function matchArray(targetArray, searchValues, fromIdx) {
  for (let i = 0, len = searchValues.length; i < len; ++i) {
    if (targetArray[fromIdx + i] !== searchValues[i]) {
      return false;
    }
  }
  return true;
}

//    "/Registry (Adobe) /Ordering (Japan1) /Supplement [0-9]"
// => "/Registry(Adobe) /Ordering(Identity) /Supplement 0"
function convertPdf(arrayBuffer) {
  const bytes = new Uint8Array(arrayBuffer);

  let currentIdx = 0;
  let maxIdx = bytes.length - FROM_CHAR_CODES.length - 1;
  while (currentIdx <= maxIdx) {
    let lastByte = bytes[currentIdx + FROM_CHAR_CODES.length];
    if (matchArray(bytes, FROM_CHAR_CODES, currentIdx) && lastByte >= 48 && lastByte <= 57) {
      for (let i = 0, len = TO_CHAR_CODES.length; i < len; ++i) {
        bytes[currentIdx + i] = TO_CHAR_CODES[i];
      }
      bytes[currentIdx + TO_CHAR_CODES.length] = 48;
      currentIdx += TO_CHAR_CODES.length + 1;
    } else {
      ++currentIdx;
    }
  }

  return bytes;
}

function appendConvertedPdfLink(file) {
  const reader = new FileReader();
  reader.addEventListener("load", async (evt) => {
    const bytes = convertPdf(reader.result);
    const div = document.createElement('div');
    const a = document.createElement('a');
    a.innerHTML = `Open converted ${file.name}`;
    // cf. https://stackoverflow.com/questions/31246204/convert-canvas-to-image-then-download
    a.href = URL.createObjectURL(new Blob([bytes], { type: 'application/pdf' }));;
    a.target = '_blank';
    div.appendChild(a);
    document.getElementById('converted-pdfs').appendChild(div);
  });
  reader.readAsArrayBuffer(file);
}

function onDrop(evt) {
  evt.preventDefault();
  const files = evt.dataTransfer.files;
  for (let i = 0, len = files.length; i < len; ++i) {
    appendConvertedPdfLink(files[i]);
  }
}

document.addEventListener('DOMContentLoaded', (evt) => {
  const dropArea = document.getElementById('droparea');
  dropArea.innerHTML = 'ここにドラッグ & ドロップ';
  dropArea.addEventListener('dragover', evt => evt.preventDefault());
  dropArea.addEventListener('drop', onDrop);
});