jQuery.ajax() does not have a responseType setting by default. You can use a polyfill, for example jquery-ajax-blob-arraybuffer.js which implements binary data transport, or utilize fetch().
Note also, chrome, chromium have issues displaying .pdf at either <object> and <embed> elements, see Displaying PDF using object embed tag with blob URL, Embed a Blob using PDFObject. Substitute using <iframe> element for <object> element.
$(function() {
  var pdfsrc = "/display";
  var jQueryAjaxBlobArrayBuffer = "https://gist.githubusercontent.com/SaneMethod/" 
                         + "7548768/raw/ae22b1fa2e6f56ae6c87ad0d7fbae8fd511e781f/" 
                         + "jquery-ajax-blob-arraybuffer.js";
  var script = $("<script>");
  $.get(jQueryAjaxBlobArrayBuffer)
  .then(function(data) {
    script.text(data).appendTo("body")
  }, function(err) {
    console.log(err);
  })
  .then(function() {
    $.ajax({
      url: pdfsrc,
      dataType: "arraybuffer"
    })
    .then(function(data) {
      // do stuff with `data`
      console.log(data, data instanceof ArrayBuffer);
      $("#pdfviewer").attr("src", URL.createObjectURL(new Blob([data], {
            type: "application/pdf"
          })))
     }, function(err) {
          console.log(err);
     });
  });
});
Using fetch(), .arrayBuffer()
  var pdfsrc = "/display";
  fetch(pdfsrc)
  .then(function(response) {
    return response.arrayBuffer()
  })
  .then(function(data) {
    // do stuff with `data`
    console.log(data, data instanceof ArrayBuffer);
    $("#pdfviewer").attr("src", URL.createObjectURL(new Blob([data], {
        type: "application/pdf"
    })))
  }, function(err) {
      console.log(err);
  });
plnkr http://plnkr.co/edit/9R5WcsMSWQaTbgNdY3RJ?p=preview 
version 1 jquery-ajax-blob-arraybuffer.js, jQuery.ajax(); version 2 fetch(), .arrayBuffer()