When you read or write to a file Windows will often keep some or all of the file resident in memory (in the Standby List). So that if it is needed again, it is just a soft-page fault to map it into the processes' memory space.
The algorithm for what pages of a file will be kept around (and for how long) isn't publicly documented. So the short answer is that if you are lucky some or all of it may still be in memory. You can use the SysInternals tool VMmap to see what of your file is still in memory during testing.
If you want to increase your chances of the data remaining resident, then you should use Memory Mapped Files to pass the data between the two processes.
Good reading on Windows memory management: Mysteries of Windows Memory Management Revealed