julien,
I've experimented a bit more with the "Fit Width" zoom type of Acrobat, and it doesn't seem like this is a setting that are specific to every pdf files. Rather, it's a setting in the Acrobat program itself, and it sets it to the default view mode every time you open the application.
I did a test, and generated a pdf. I opened it, the mode was actually "Fit Width". I've changed it to "Actual Size", then did a save as. When I opened it again, it was back to Fit Width. Further experimentation with the various acrobat scripting commands revealed that the zoom type is a property of the view, not the document.
The passthrough command that you were using, if you look at it, used to set the Document's view when you open the document, but it's not a property of the pdf itself.
To change the default view mode, you can go to Edit->Preferences>Page Display, and set the default zoom to what you want. All pdf files you open will use that mode, so you can set it to Fit Width, and all pdf files will be opened "Fit Width". Note that this is under Acrobat 6.0, the newer versions might have a difference Preference menu.
That said, I cannot explain why the passthrough no longer works in version 7, other than this is probably due to the change in the pdf libraries, and the way pdf are created in general.
Hope that helps.
Regards,
Rapha