In the new version 9 of Adobe Acrobat it is not possible -by design- to open multiple documents ion the same window (MDI). Mac users may not know what I am talking about, since this feature is on Windows only.
While web browsers have evolved to Tabbed document interface, Adobe is making a step back cluttering our monitors with multiple windows of the same application (SDI). The reasons argued include Feature Parity with Macintosh, reduction of debugging costs, and advise from Microsoft (haha!).
The response of complaining users have been overwhelming and the Product Manager have closed the comments in the Adobe Blog entry saying "We're heard you loud and clear [...] there's no need to beat a dead horse".
Let's hope that Sumatra PDF, one of the current open source pdf viewers, implements tabbed view before Adobe does. It was a silly move to begin with, but wielding "advise from Microsoft" as an argument...
After comments and other forum post I can add that :
Sumatra developers agree with Adobe, and don't plan to implement MDI.
Not open source, but commercial with a free version, PDF XChange viewer seems to be an option I will be trying now.