Need help downloading a pdf file on android with PhoneGap

I have a small programm to take some text inputs and a signature as a png file in a phonegap appliaction. I take the png input with jSignature and add it to the pdf file with jsPDF.

I managed to download the file on my PC, but with the PhoneGap Developer mobile application, it just does nothing.

Code is basically like:

<... all the html with the forms etc ...>

function saveSignature(){
        var data_signature = $("#signature").jSignature("getData", "image");
        $("#signature").jSignature("reset");

        doc = new jsPDF();
        doc.setFontSize(20);
        doc.text("test", 30, 20)
        doc.addImage(data_signature[1], 'PNG', 15, 40, 50, 29);

       doc.save()
       alert("done");
    }

the data_signature is an array with the PNG file as a base64 string, the rest should be self-explaining.