I want to create such a button on my portfolio website so that if someone clicks this button he/she can see my resume by pdf viewer and also download this pdf resume. how can I do it by vanilla javascript?
You could convert your PDF content to base64 string.
And then you could do something like this.
const content = 'data:application/pdf;base64,<base64 PDF content string>';
const linkSource = content;
const downloadLink = document.createElement('a');
const fileName = 'name.pdf';
downloadLink.href = linkSource;
downloadLink.download = fileName;
downloadLink.click();