The marked answer doesn't really answer the question. I'll tell you how, but I want you to know first that Bluetooth on most mobile platforms is an inherently insecure technology that allows for malicious third parties to tamper with data, violate privacy,
and place files on your computer. Windows gets around this insecurity (in typical Windows fashion) by making the user establish a complex chain of trust to prevent malicious activity. But here is what you wanted to know:
First thing you need to do is pair your Surface with your iPhone. You can do this by clicking the Settings charm and selecting "Change PC Settings" at the bottom. In the Settings app, click "Devices." Make your iPhone visible via Bluetooth, and then select
the name you gave to your Surface computer on your iPhone. The pairing should be very trivial after that.
Now, on the Desktop, click the expandable icon list in the notification area. In the default configuration, this will be on the right side of your task bar. You should find a little symbol shaped like the letter B, this is the standard symbol for Bluetooth
technology. Right-click/long-press on this icon. When the context-menu appears, select "Receive a file." The Bluetooth window should pop up.
Now, share the file on your iPhone with the Surface, and the download should begin. After the download completes, you should have the chance to choose where you want the file saved in a browse menu.
Cheers!