Automatically "download" a pdf you retrieved from api.

xhr-file-download.js

let url = 'https://<your pdf endpoint>';
let filename = 'File.pdf';

let xhr = new XMLHttpRequest();
xhr.responseType = 'blob'; // set response type to blob
xhr.onreadystatechange = () => {
  if (xhr.readyState == 4 && xhr.status == 200) {
    
    // create a blob with the content in XHR's response:
    let blob = new Blob([xhr.response], { type: 'application/pdf' });
    
    // create an invisible HTML anchor tag:
    let downloadLink = document.createElement('a');
    document.body.appendChild(downloadLink);
    downloadLink.style.display = 'none';

    // create URL to your local blob:
    let url = window.URL.createObjectURL(blob);
    downloadLink.href = url;
    
    // set the file name to download:
    downloadLink.setAttribute('download', filename);
    
    // anonimously click the invisible link you created:
    downloadLink.click();
    
    // clean out the HTML as soon as the download starts:
    document.body.removeChild(downloadLink);
    window.URL.revokeObjectURL(url);
  }
}

xhr.open("POST", url);
xhr.setRequestHeader("cache-control", "no-cache");
xhr.send(data);

/* This is based on that great article from Alex Hadik I found here: https://www.alexhadik.com/writing/xhr-file-download/ */