加拿大网签Letter of Explanation 上传格式问题？？？

求大神教教加拿大网签申请文件最后的letter of explanation 要怎么上传，上传了说要pdf文件，上传了pdf文件有说什么You have submitted an invalid form. Please complete and validate an electronic version of the application form, save it to your computer, and upload it.
Note: You cannot submit an application form that has been scanned.You do not need to sign your application form. You must provide a digital signature once you are ready to transmit your completed application to CIC.这是什么意思啊。。。。。不知道怎么办了知道的大神帮帮忙啊。。。