You can't. This is no longer possible to run in that mode.
In old versions of Acrobat (e.g. 5), all documents opened in the same Acrobat window - at least on Windows. Adobe later decided to unify the Mac and Windows version by adding the then new mode where every document opens in a new window. WIth Acrobat 9 (this was an option back then). With Acrobat 9 this was made the only way to display PDF files.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com