题主贴出的demo里直接下载到用户选择的路径，不能保证下载成功。
webDownloadItem默认路径在应用沙箱的web目录内，用户无法查看。

1. 如果想通过文件管理查看，一种是将下载路径修改到有访问权限的Download目录，请参考使用Web组件发起一个下载任务的使用DocumentViewPicker()那个示例。这种方式不会弹出模态框选择保存路径。关键代码：

documentSaveOptions.pickerMode = picker.DocumentPickerMode.DOWNLOAD

1. 如果想让用户选择可查看的目录，可通过先下载至沙箱，下载完复制到用户选择的路径。可以不设置documentSaveOptions.pickerMode。完整示例：

import { webview } from '@kit.ArkWeb';
import { BusinessError } from '@kit.BasicServicesKit';
import { picker, fileUri, fileIo, statfs } from '@kit.CoreFileKit';
import { common } from '@kit.AbilityKit';
import { fileManagerService } from '@kit.FileManagerServiceKit';

@Entry
@Component
struct WebComponent {
  controller: webview.WebviewController = new webview.WebviewController();
  delegate: webview.WebDownloadDelegate = new webview.WebDownloadDelegate();
  private delegateSet: boolean = false;
  abilityContext = this.getUIContext().getHostContext() as common.UIAbilityContext;
  downloadFilePath: string = '';
  saveFilePath: string = '';

  onPageLoadEnd(): void {
    if (!this.delegateSet) {
      this.setupDownloadDelegate();
      this.delegateSet = true;
    }
  }

  setupDownloadDelegate(): void {
    try {
      this.delegate.onBeforeDownload(async (webDownloadItem: webview.WebDownloadItem) => {
        let desiredFileName = webDownloadItem.getSuggestedFileName();

        this.saveFilePath = await this.selectSavePathWithPicker(desiredFileName);
        if (this.saveFilePath.length == 0) {
          console.warn("用户取消选择");
          return;
        }
        console.info(`准备下载到: ${this.saveFilePath}`);

        this.downloadFilePath = this.abilityContext.cacheDir + '/' + desiredFileName; //缓存下
        console.info(`准备缓存到: ${this.downloadFilePath}`);

        webDownloadItem.start(this.downloadFilePath);
        console.info("webDownloadItem.start() 已调用");
      });

      this.delegate.onDownloadUpdated((item: webview.WebDownloadItem) => {
        console.info(`下载进度 - guid: ${item.getGuid()}, 百分比: ${item.getPercentComplete()}%`);
      });

      this.delegate.onDownloadFailed((item: webview.WebDownloadItem) => {
        console.error(`下载失败 - guid: ${item.getGuid()}, error: ${item.getLastErrorCode()}`);
      });

      this.delegate.onDownloadFinish((item: webview.WebDownloadItem) => {
        console.info(`下载完成 - guid: ${item.getGuid()}`);
        this.copyFile();
      });

      this.controller.setDownloadDelegate(this.delegate);
      console.info("DownloadDelegate 设置成功");
    } catch (error) {
      const e = error as BusinessError;
      console.error(`设置代理失败: ${e.code} - ${e.message}`);
    }
  }

  copyFile(): boolean {
    try {
      let freeSize = statfs.getFreeSizeSync(this.saveFilePath);
      if (freeSize < fileIo.statSync(this.downloadFilePath).size) { // 判断剩余空间是否小于文件大小
        return false;
      }
      let srcFile = fileIo.openSync(this.downloadFilePath);
      let destFile = fileIo.openSync(this.saveFilePath, fileIo.OpenMode.READ_WRITE | fileIo.OpenMode.CREATE)
      fileIo.copyFile(srcFile.fd, destFile.fd).then(() => {
        fileIo.unlinkSync(this.downloadFilePath);
        this.getUIContext().getPromptAction().showToast({message:'下载保存完成'});
      }).catch(() => {
        fileManagerService.deleteToTrash(this.saveFilePath).catch(() => {
          // TODO: Implement error handling.
        });
        return false;
      }).finally(() => {
        fileIo.closeSync(srcFile.fd);
        fileIo.closeSync(destFile.fd);
      })
    } catch (error) {
      return false;
    }
    return true;
  }

  /** 使用 Picker 让用户选择保存路径和文件名 */
  async selectSavePathWithPicker(fileName: string): Promise<string> {
    try {
      const options = new picker.DocumentSaveOptions();
      options.newFileNames = [fileName]

      const documentPicker = new picker.DocumentViewPicker();
      const result: Array<string> = await documentPicker.save(options);

      const tmpUri = result[0];
      if(!tmpUri){
        return '';
      }
      const uri = new fileUri.FileUri(tmpUri);
      return uri.path;
    } catch (err) {
      const e = err as BusinessError;
      console.error(`Picker 选择失败: ${e.code} - ${e.message}`);
      return '';
    }
  }

  build() {
    Column() {
      Web({
        src: "https://developer.huawei.com/consumer/cn/blog/topic/03214846431435058",
        controller: this.controller
      })
        .width('100%')
        .height('100%')
        .onPageEnd((data)=>{
          this.onPageLoadEnd();
        })
    }
    .width('100%')
    .height('100%')
  }
}