When your app needs more memory than you physically have some other memory will get swapped out into the page file. That is your first increase in page file usage. After you closed your app that memory is available again and other processes will get their memory pages swapped back into physical memory. That should be why you see page file usage a while after your app was closed.
Also, some memory is used to cache disk data so it can be used faster when accessed again. The cache manager will also need some time to decide that parts from your large text file are not worth caching any more. I am not aware if you can control the amount or time taken for the cache.
In the end I guess you just have to continue using the system a while after your app was closed to see a decrease in page file usage - assuming that the remaining apps do not need more memory than physically available.