加签网签上传文件问题

需要上传的资料都扫描成pdf格式了，但是上传时就提示如下错误：Validation error: You have not completed the mandatory fields or you have not completed them correctly. Please make corrections where indicated.
Error Description:
Document is not a valid PDF file
有说是浏览器问题，但是试了IE、Google、搜狗等都不行，求教啊