I have created a database \"test\" with some tables in MS SQL Server 2008 R2 (i.e. MS SQL Server Management Studio).
Now, I need to export this database as a MDF file.>
If what you want is a replica of your SQLEXPRESS/MDF local file- on a remote server (SQL Server 2005)
You can right click in the "Server Explorer" on your db.mdf file and press "Publish to provider..."
You can choose a variety of compatibilities with SQL Server 2005, 2008, etc.
The output is a .sql query file ..
Came across this question and thought this would be a good alternative answer, since I got here by searching for the exact same thing!