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 (this._currentPageNumber === val) {
      if (resetCurrentPageView) {
        this._resetCurrentPageView();
      }
      return true;
    }
    if (!(0 < val && val <= this.pagesCount)) {
      return false;
    }
    const previous = this._currentPageNumber;
    this._currentPageNumber = val;
    this.eventBus.dispatch("pagechanging", {
      source: this,
      pageNumber: val,
      pageLabel: this._pageLabels && this._pageLabels[val - 1],
      previous,
    });
    if (resetCurrentPageView) {
      this._resetCurrentPageView();
    }
    return true;
  }
  /**
   * @type {string|null} Returns the current page label, or `null` if no page
   *   labels exist.
   */
  get currentPageLabel() {