How do you open a logfile (.rdf) downloaded from an EV3 brick?
In the LEGO MINDSTORMS EV3 Education software:


Open a new experiment.
Go to the Tools menu and select Data Log File Manager.
Browse to the the .rdf file and click the Import button. (It can be on the brick or on your computer.)
Close the Data Log File Manager
On the Tools menu, select Export Datasets
Save the file as a .csv file
Open the .csv file in Excel