How to view a PDF with more than 2 pages side by side?

Use Wrapped Scrolling option in Mozilla Firefox's built-in PDF viewer.

1. Open your PDF file in Mozilla Firefox browser.
2. On PDF document toolbar (above the document view) open menu >>. See the screenshot.
3. Select Wrapped Scrolling menu item.
4. Zoom your document out to see as many pages as it fits into width of the view.

Use Horizontal Scrolling option in default Firefox browser PDF viewer PDF.js

PDF.js is also available as a plugin for other browsers.

1. Open your PDF file in Mozilla Firefox browser
2. On the PDF document toolbar (top right above the document view) click on >>
3. Select Horizontal Scrolling

miroxlav already pointed to the use of PDF.js but I think that horizontal scrolling is a more appropriate answer because the question asked about side by side views.