Root Object missing or invalid

Lori: I just upgraded from 5.0 to 8.0 and many of our office PCs just have Reader 6.0. First, everything I do in 8.0 is defaulting to require 7.0 or higher and I can't find the settings to change it. Another very important document I just rolled out gives them an error even if opening with Reader 7.0, that says "the root object is missing or invalid". We are in great need of this document as many in the company use it and we are all stuck at what to do. Any help would be very much appreciated.
 
Kim Mynatt
865-380-3000 Ext. 5680 or email: [link=mailto:kim [dot] mynatt [at] vmf [dot] com]kim [dot] mynatt [at] vmf [dot] com[/link]