🚀 go-pugleaf

Here is the link to why this option isn't there any more:
 <http://blogs.adobe.com/acrobat/2008/09/mdi_vs_sdi_in_acrobat.html>

Beyond what everyone else said in that blog, being able to open documents inside a single window allows you to flip back and forth between two pages in different documents using the "Previous View" and "Next View" buttons. Now, you can get the same effect if you have two Adobe Reader windows open and have them positioned side-by-side so that you can look from one window to the other, but if you're not running a dual-monitor system, you won't have enough real estate on your monitor to do this effectively.

The "Previous View" and "Next View" buttons allow for maximum monitor real estate usage and lets you magnify each section of both documents so that they can be at a level that is readable.

In the TechNote that is found at <http://kb.adobe.com/selfservice/viewContent.do?externalId=kb405108&sliceId=2>, they give you the work-around of using the "Group similar taskbar buttons", but if you're still running Windows 2000, that doesn't help you.