Possibly a better solution is to NOT submit the form. You could use javascript to construct a GET url and use an iframe.
eg:
<form>
<input id="someText" />
<button onclick="downloadPdf()">Click Me</button>
</form>
<script>
function downloadPdf() {
var url = "/path/to/pdf?someText=" + encodeURIComponent($("#someText").val());
startThrobber();
$('<iframe src="' + url + '" onLoad="stopThrobbber()">').appendTo('body');
}
</script>