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
if (file.name) {
        url = { url, originalUrl: file.name };
      }
      PDFViewerApplication.open(url);
    } else {
      PDFViewerApplication.setTitleUsingUrl(file.name);
      
      const fileReader = new FileReader();
      fileReader.onload = function webViewerChangeFileReaderOnload(event) {
        const buffer = event.target.result;
        PDFViewerApplication.open(new Uint8Array(buffer));
      };
      fileReader.readAsArrayBuffer(file);
    }
    
    const appConfig = PDFViewerApplication.appConfig;
    appConfig.toolbar.viewBookmark.setAttribute("hidden", "true");
    appConfig.secondaryToolbar.viewBookmarkButton.setAttribute(
      "hidden",
      "true"
    );
    appConfig.toolbar.download.setAttribute("hidden", "true");
    appConfig.secondaryToolbar.downloadButton.setAttribute("hidden", "true");
  };
  webViewerOpenFile = function (evt) {
    const openFileInputName = PDFViewerApplication.appConfig.openFileInputName;
    document.getElementById(openFileInputName).click();
  };
}
function webViewerPresentationMode() {