HTML
<a id="link1" href="sample/smp1.zip" download="smp1.zip">smp1.zip</a>
<a id="link2" href="sample/smp2.zip" download="smp2.zip">smp2.zip</a>
<a id="link3" href="sample/smp3.png" download="smp3.png">smp3.png</a>
<a id="link4" href="sample/smp4.txt" download="smp4.txt">smp4.txt</a>
<input type="button" value="複数ファイルダウンロード" onclick="test1()" class="btn btn-success btn-sm" />
JavaScript
function test1(){
$('#link1')[0].click();
$('#link2')[0].click();
$('#link3')[0].click();
$('#link4')[0].click();
}