I use the following in Managed Preferences (using underscores to show indentation):
Display Name: Default applications
Description: Sets preferred applications.
Apply Setting To: User Level At Every Login
Domain: ~/Library/Preferences/com.apple.LaunchServices
Key: LSHandlers
Value:
<array>
<dict>
_____<key>LSHandlerContentType</key>
____<string>com.adobe.pdf</string>
____<key>LSHandlerRoleAll</key>
____<string>com.adobe.acrobat.pro</string>
</dict>
<dict>
____<key>LSHandlerContentTag</key>
____<string>pdf</string>
____<key>LSHandlerContentTagClass</key>
____<string>public.filename-extension</string>
____<key>LSHandlerRoleAll</key>
____<string>com.adobe.acrobat.pro</string>
_</dict>
</array>
This includes two examples. The first key is LSHandlerContentType, which assigns Acrobat Pro to open PDF files based on file type. The second key is LSHandlerContentTag, which assigns Acrobat Pro to open PDF files based on file extension. Use either or both. Shouldn't make a difference.