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
}
        generatorId = generator.replace(/[ .-]/g, "_");
        return true;
      });
    }
    let formType = null;
    if (info.IsXFAPresent) {
      formType = "xfa";
    } else if (info.IsAcroFormPresent) {
      formType = "acroform";
    }
    this.externalServices.reportTelemetry({
      type: "documentInfo",
      version: versionId,
      generator: generatorId,
      formType,
    });
    this.eventBus.dispatch("metadataloaded", { source: this });
  },
  /**
   * @private
   */
  async _initializePageLabels(pdfDocument) {
    const labels = await pdfDocument.getPageLabels();
    if (pdfDocument !== this.pdfDocument) {
      return; 
    }
    if (!labels || AppOptions.get("disablePageLabels")) {
      return;
    }