【问题】
直接new一个PdfWriter的对象直接会报错：
iText.Kernel.Exceptions.PdfException: 'Unknown PdfException.'
NotSupportedException: Either com.itextpdf:bouncy-castle-adapter or com.itextpdf:bouncy-castle-fips-adapter dependency must be added in order to use BouncyCastleFactoryCreator
【解决方法】
nuget管理器里面下载itext7.bouncy-castle-adapter解决了问题。