TypeError：res.blob不是函数

我想使用这个Angular 6代码实现文件下载：

休息API：

private static final Logger LOG = LoggerFactory.getLogger(DownloadsController.class);

@GetMapping(path="export") 
public ResponseEntity<byte[]> export() throws IOException {
    File pdfFile = Paths.get(EXTERNAL_FILE_PATH).toFile();


    byte[] fileContent = Files.readAllBytes(pdfFile.toPath());

    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.parseMediaType("application/pdf"));
    // Here you have to set the actual filename of your pdf
    String filename = "output.pdf";
    headers.setContentDispositionFormData(filename, filename);
    headers.setCacheControl("must-revalidate, post-check=0, pre-check=0");
    ResponseEntity<byte[]> response = new ResponseEntity<>(fileContent, headers, HttpStatus.OK);
    return response;
}

服务：

import {Injectable} from '@angular/core';
import {HttpClient, HttpParams} from "@angular/common/http";
import {Observable} from "rxjs/index";
import {environment} from "../../../environments/environment";
import {HttpUtils} from "../common/http-utils";
import { map } from 'rxjs/operators';
import {Http, ResponseContentType} from '@angular/http';


@Injectable({
  providedIn: 'root'
})
export class DownloadService {

  constructor(private http: HttpClient) {
  }

  downloadPDF(): any {
    return this.http.get(environment.api.urls.downloads.getPdf, {
        responseType: 'blob'
      })
      .pipe(
        map((res: any) => {
          return new Blob([res.blob()], {
            type: 'application/pdf'
          })
        })
      );
    }  
}

零件：

import {Component, OnInit} from '@angular/core';
import {DownloadService} from "../service/download.service";
import {ActivatedRoute, Router} from "@angular/router";
import {flatMap} from "rxjs/internal/operators";
import {of} from "rxjs/index";
import { map } from 'rxjs/operators';

@Component({
  selector: 'app-download',
  templateUrl: './download.component.html',
  styleUrls: ['./download.component.scss']
})
export class DownloadComponent implements OnInit {

  constructor(private downloadService: DownloadService,
              private router: Router,
              private route: ActivatedRoute) {
  }

  ngOnInit() {   
  }

  export() {               
    this.downloadService.downloadPDF().subscribe(res => {
      const fileURL = URL.createObjectURL(res);
      window.open(fileURL, '_blank');
    });         
  } 
}

该文件存在于目录中，但当我尝试下载时，我收到错误：

ERROR TypeError: res.blob is not a function
    at MapSubscriber.project (download.service.ts:24)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/map.js.MapSubscriber._next (map.js:35)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:54)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/map.js.MapSubscriber._next (map.js:41)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:54)
    at FilterSubscriber.push../node_modules/rxjs/_esm5/internal/operators/filter.js.FilterSubscriber._next (filter.js:38)
    at FilterSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:54)
    at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber.notifyNext (mergeMap.js:84)
    at InnerSubscriber.push../node_modules/rxjs/_esm5/internal/InnerSubscriber.js.InnerSubscriber._next (InnerSubscriber.js:15)
    at InnerSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:54)

你知道我怎么解决这个问题吗？我是否需要添加其他配置才能通过Angular Web UI下载文件？

我使用spring-boot-starter-parent版本2.1.0.RELEASE和angular 6

更新：我测试了这个，但没有任何反应：

downloadPDF(): any {
    return this.http.get(environment.api.urls.downloads.getPdf, {
        responseType: 'blob'
      })
      .pipe(
        map((res: any) => {
          return new Blob([res], {
            type: 'application/pdf'
          })
        })
      );
    }