"Reduce File Size" won't make a PDF readable in Acrobat 5 if you don't choose Acrobat 5 when using the feature. (There is a drop-down list of options in the dialog box that springs up.) If you want to preserve the layers -- which, again, we do like -- you choose Acrobat 6. If you choose Acrobat 5, it obliterates the layers; it just so happens that PDF layers are incompatible with Acrobat 5, and, again, it's the layers that are causing the problem with Acrobat 5.
The layers are there for a couple of reasons.
The layers provide users with options to print whatever elements of the PDF suit their fancy/printer/budget, as constrained by the assignment of certain elements to certain layers -- but I set things up so that even the illustrations can be hidden to save ink/toner.
The layers let EN Publishing offer a single file that contains full-blown and bare-bones versions of the book (and several versions in between), rather than two or three files, which reduces download size and eliminates the possibility of opening the "wrong" file -- seems to me you'd need about six different Acrobat 5 PDFs to match the flexibility of one layered PDF.
The layers make for a neat "toy" the user can play with in Reader 6 or 7.
All in all, it sounded like a value-add to me. So after getting software that supported writing PDF layers and working the process out, I thought having the layers was smashingly cool. So that's the thinking behind going ahead and using the Acrobat 6 layers.
[aside] Of course, I'm always willing to revisit a choice like that for whatever reason. I'm not insensitive to issues with incompatibility and so forth, for example, and it certainly doesn't please me to offer something some of ENP's customers can't use, however highly I think of what I accomplished. (Obviously *I* think it's cool.

) So trust me when I say there has been some heavy thinking and a lot of conversation on this very issue. And as if that weren't enough, a technical glitch forced Fantastic Science into Acrobat 5, so now we have this experiment floating around out there and we'll see what people say once the reviews start rolling in. [/aside]
Erm... does that answer the question?