切换主题
https://guru.kevin2li.top/docs/pdf-toolbox/pdf-watermark/remove#步骤一识别水印色阶
https://www.bilibili.com/video/BV11j411q74U/?spm_id_from=333.999.0.0&vd_source=5b3cbd998dbd36fcae99c3190d19c399
87900