Save pdf in App. From a JSON with base64. Cordova Phonegap File or File transfer. writer.truncate (11) writer.seek (4)

How to convert base64 to pdf?
It is right? The phone gives error message when opening.

In the gotFileWriter(writer) function that are for the numbers that are passed as parameters writer.truncate(11) writer.seek(4)

function base64Tobinaty(jsonMasImg) {
    var base64str = jsonMasImg.DocArchivoB64;

    var binary = atob(base64str.replace(/\s/g, ''));
    var len = binary.length;
    var buffer = new ArrayBuffer(len);
    var view = new Uint8Array(buffer);
    for (var i = 0; i < len; i++) {
        view[i] = binary.charCodeAt(i);
    }
    // create the blob object with content-type "application/pdf"               
    var blob = new Blob([view], {type: "application/pdf"});
    var url = URL.createObjectURL(blob);
    return blob;
}


var documentoDescargado = null;

function descargarDocumento(jsonMasImg) {
    documentoDescargado = jsonMasImg; // obtengo un objeto binario y
    window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, gotFS, fail);
}

function gotFS(fileSystem) {
    var nombreDoc = documentoDescargado.DocArchivoNombre;
    alert(nombreDoc);
    fileSystem.root.getFile( nombreDoc, {create: true, exclusive: false}, gotFileEntry, fail);

}

function gotFileEntry(fileEntry) {
    fileEntry.createWriter(gotFileWriter, fail);
}

function gotFileWriter(writer) {
    var pdf =  base64Tobinaty(documentoDescargado);

    writer.onwriteend = function (evt) { writer.truncate(); writer.onwriteend = function (evt) { writer.seek(); writer.write( ); writer.onwriteend = function (evt) {}; }; };
    alert("paso el onwriteend");
    writer.write(pdf);
}

function fail(error) {
    alert(error.code);
}