There is no way to download multiple files through pure HTML. You could perhaps use javascript to download these files:
var links = ['file1.pdf', 'file2.jpg'] // Change with file paths
for (let filename in links) {
var element = document.createElement('a');
element.setAttribute('href', '/'+links[filename]);
// Change 'file' to something else to change the name of the downloaded file
element.setAttribute('download', 'file');
// Optionally, hide the element before appending, so that the user doesn't download the file again.
document.body.appendChild(element)
element.click() // This will prompt the user to download
}