1、首先将下载好的文件，保存到本地目录，这个目录是用户看不到的；

2、然后通过picker的save保存文件，这个picker，它只是获取公共目录uri用的

3、当picker有回调时，将公共目录的uri获取之后，把下载好的文件复制到公共路径下，

公共路径=uris1[0]+'/'+'56151651.pdf'

  async saveSystemFile(name:string,path:string){
    try {
      let documentSaveOptions = new picker.DocumentSaveOptions();
      documentSaveOptions.pickerMode=picker.DocumentPickerMode.DOWNLOAD
      documentSaveOptions.fileSuffixChoices = ['文档|.txt', '.pdf'];
      documentSaveOptions.newFileNames = [name];
      let documentPicker = new picker.DocumentViewPicker(getContext() as common.Context);
      documentPicker.save(documentSaveOptions).then((uris1: Array<string>) => {

        let uri = uris1[0]+'/'+name;

        let srcDirUriLocal: string = fileUri.getUriFromPath(path);

        fs.copy(srcDirUriLocal, uri, (err: BusinessError) => {
          if (err) {
            console.error(`Failed to copy: ${JSON.stringify(err)}`);
            return;
          }
        })

      }).catch((err: BusinessError) => {
        console.error('DocumentViewPicker.save failed with err: ' + JSON.stringify(err));
      });
    } catch (error) {
      let err: BusinessError = error as BusinessError;
      console.error('DocumentViewPicker failed with err: ' + JSON.stringify(err));
    }
  }