如何在pdf flutter中添加复选框？syncfusion_flutter_pdf：^20.3.58Flutter版本为3.0.2
请告诉我syncfusion表单是否不是免费的。
下面的代码是不可见的pdf i genarate.

document.form.fields.add(PdfCheckBoxField(
document.pages.add(), 'CheckBox', Rect.fromLTWH(100, 200, 70, 45),
highlightMode: PdfHighlightMode.push,
borderStyle: PdfBorderStyle.dot,
borderColor: PdfColor(230, 0, 172),
backColor: PdfColor(153, 255, 102),
foreColor: PdfColor(255, 153, 0),
borderWidth: 1,
style: PdfCheckBoxStyle.diamond,
isChecked: true));

source: https://help.syncfusion.com/flutter/pdf/working-with-forms#adding-the-check-box-field