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
filename = this._docFilename;
    
    
    if (!this.pdfDocument || !this.downloadComplete) {
      this.download({ sourceEventType });
      return;
    }
    this._saveInProgress = true;
    await this._scriptingInstance?.scripting.dispatchEventInSandbox({
      id: "doc",
      name: "WillSave",
    });
    this.pdfDocument
      .saveDocument(this.pdfDocument.annotationStorage)
      .then(data => {
        const blob = new Blob([data], { type: "application/pdf" });
        downloadManager.download(blob, url, filename, sourceEventType);
      })
      .catch(() => {
        this.download({ sourceEventType });
      })
      .finally(async () => {
        await this._scriptingInstance?.scripting.dispatchEventInSandbox({
          id: "doc",
          name: "DidSave",
        });
        this._saveInProgress = false;
      });
  },
  downloadOrSave(options) {