This problem applies to all the documents that I have ever opened.

Configuration:
- Web browser and its version: Google Chrome 83.0.4103.61 , also Firefox 76.0.1
- Operating system and its version: Win 7 x64
- PDF.js version: 2.3.164 version of extension
- Is a browser extension: yes

Steps to reproduce the problem:
1. Select any "Spreads" mode of view. Or "No spreads", but wrapped scrolling mode - doesn't matter. 
2. Select "Page Fit" zoom mode. We should get several pages per screen
3. Try to scroll document by any of keys: Page Down, n, j, Space Bar

I expect to see the next group of pages (for example, if there are 5 pages on the screen, I expect to see the next 5 pages)

What went wrong? Scrolling goes one page at a time, so if I have 5 pages on the screen, nothing happens when I pressed Page Down 5 times . For the 6th time I finally see the next group of pages.

Just for example: Adobe Reader scrolls properly in two-page mode, 2 pages at once. 

P.S. Sorry for my English, I hope I correctly stated the essence of the problem
function downloadByUrl() {
      downloadManager.downloadUrl(url, filename);
    }
    const downloadManager = this.downloadManager,
      url = this.baseUrl,
      filename = this._docFilename;
    
    
    if (!this.pdfDocument || !this.downloadComplete) {
      downloadByUrl();
      return;
    }
    this.pdfDocument
      .getData()
      .then(function (data) {
        const blob = new Blob([data], { type: "application/pdf" });
        downloadManager.download(blob, url, filename, sourceEventType);
      })
      .catch(downloadByUrl); 
  },
  async save({ sourceEventType = "download" } = {}) {
    if (this._saveInProgress) {
      return;
    }
    const downloadManager = this.downloadManager,
      url = this.baseUrl,