c# 设置水印，消除水印

方案1：

图像处理

opencv etc

方案2：

开源框架，直接使用，已经优化

https://github.com/itext/itextsharp
https://github.com/itext/itext7-dotnet
https://github.com/itext/i7n-pdfsweep

https://github.com/itext/itextsharp/releases/tag/5.5.13