How to set filename for base64 rendered pdf using object tag?

please try this

const FILE_NAME = 'myfile.pdf';
const file_header=";headers=filename";

fetch('https:your-url/myfile.pdf?dl=0').then(r => r.blob())
.then(blob=>{
  const f = new FileReader();
  f.onload = () => myPdfViewer.src = f.result.replace(';', file_header + encodeURIComponent(FILE_NAME) + ';');
  f.readAsDataURL(blob);
  });

Then insert id myPdfViewer to an iframe.
I hope it can help.