<a-upload
  name="file"
  @change="handleChange"
  accept='.pdf' :file-list="file" :before-upload="beforeUploadFile"
>
  <a-button> <a-icon type="upload" /> 请选择上传文件 </a-button>
</a-upload>

beforeUploadFile (f, l) {
      const isLt50M = f.size / 1024 / 1024 < 50
      if(!isLt50M) {
        this.$message.error(f.name + '文件大小超出限制，请修改后重新上传')
        return false
      } else {
        return true
      }
    },