<form id="fileForm">
  <div>
    <label for="pdf_file">File Upload</label>
      <input type="file" name="pdf_file" />
  </div>
  <button type="submit">Send File</button>
</form>

document.getElementById("fileForm").addEventListener("submit", (e) => {
  e.preventDefault();
  const formData = new FormData(e);
  fetch("/", {
    method: "POST",
    body: formData
  });
});

app.post('/', async (request, response) => {
  const values = await request.formData();
  const file = values.get("pdf_file");
  const processedFile = Buffer.from(await file.arrayBuffer());
  //attach to email or do other things
};