Please try this
(function ($) {
$(document).ready(function(){
function validateEmail(email) {
const re = /^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
return re.test(email);
}
if($('.submitclass').length){
$('.submitclass').click(function(){
$email_id = $('.custom-email-field').val();
if (validateEmail($email_id)) {
var url= $(this).attr('pdf_url');
var link = document.createElement('a');
link.href = url;
link.download = url.split("/").pop();
link.dispatchEvent(new MouseEvent('click'));
}
});
}
});
}(jQuery));<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form method="post">
<div class="form-item form-type-textfield form-item-email-id form-group">
<input placeholder="please enter email address" class="custom-email-field form-control" type="text" id="edit-email-id" name="email_id" value="" size="60" maxlength="128" required />
</div>
<button type="submit" class="submitclass btn btn-danger" pdf_url="https://file-examples-com.github.io/uploads/2017/10/file-sample_150kB.pdf">Submit</button>
</form>Or use download attribute to tag in HTML5