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
"file1.pdf"
      );
      
      expect(
        getPDFFileNameFromURL("http:www.example.com/pdfs/pdf.html#file2.pdf")
      ).toEqual("file2.pdf");
    });
    it("gets correct PDF filename when multiple ones are present", function () {
      
      expect(getPDFFileNameFromURL("/pdfs/file1.pdf?name=file.pdf")).toEqual(
        "file1.pdf"
      );
      
      expect(
        getPDFFileNameFromURL("http:www.example.com/pdfs/file2.pdf#file.pdf")
      ).toEqual("file2.pdf");
    });
    it("gets PDF filename from URI-encoded data", function () {
      const encodedUrl = encodeURIComponent(
        "http:www.example.com/pdfs/file1.pdf"
      );
      expect(getPDFFileNameFromURL(encodedUrl)).toEqual("file1.pdf");
      const encodedUrlWithQuery = encodeURIComponent(
        "http:www.example.com/pdfs/file.txt?file2.pdf"
      );
      expect(getPDFFileNameFromURL(encodedUrlWithQuery)).toEqual("file2.pdf");
    });
    it("gets PDF filename from data mistaken for URI-encoded", function () {
      expect(getPDFFileNameFromURL("/pdfs/%AA.pdf")).toEqual("%AA.pdf");
      expect(getPDFFileNameFromURL("/pdfs/%2F.pdf")).toEqual("%2F.pdf");