I had a similar problem, and found a solution for Windows XP. First, I'll tell you my solution, and then I'll describe how I came up with that solution.
1) The solution is to modify your registry, specifically ...
HKEY_CLASSES_ROOT \ AcroExch.Document.7 \ shell \ Open \ command
... to point to your Reader application, instead of Acrobat.
For example, I changed the entry to the following (I'm using Reader 9.1):
@="\"C:\\Program Files\\Adobe\\Reader 9.0\\Reader\\AcroRd32.exe\" \"%1\""
If you're using Reader 8.0, then you'll need to change the directory path of course, and you should verify your actual path since it could be different.
NOTE (an alternative workaround): Before I made this mod, I could also open Reader first before printing, and the document created by doPDF would be opened in Reader instead of Acrobat.
2) How did I know this? I used Sysinternals' Process Monitor to watch what doPDF is doing within the guts of Windows during a print, and this registry value is one that doPDF is accessing on my computer to determine which application to open.
I don't know if my fix will work on your machine, but I think it'd be worth a try. If it doesn't, you might try Process Monitor to see if your machine is using a different registry key to determine which app to launch.
WARNING: Please be sure to backup your registry, or at least export HKEY_CLASSES_ROOT\AcroExch.Document.7 before modifying this key. With this key exported to a file before you make this mod, you'll have the ability to restore the registry to it's previous value.
Hope this helps!
P.S. I had tried changing the .pdf file association, but it didn't make any difference. Plus, I have a really old version of Acrobat, and have installed and updated Reader many times since Acrobat was installed. Finally, the AcroExch.Document.7 key is not accessible through the Window's File Types.