Is it possible download a file in an AIR application, store it on the user's system, and prohibit access to the files by applications other than the one with which it was downloaded?
Certainly not easily done on Windows. You might have better luck encrypting it's contents.
EDIT: Creating a file not accessible by other programs is very virus-like behavior.
Encryption is indeed probably the way to go. AIR does have a handy encrypted local store which should simplify this approach for you:
http://livedocs.adobe.com/flex/3/langref/flash/data/EncryptedLocalStore.html
Your best bet is to store the contents of the file in the EncryptedLocalStore or a local shard object. While you can store the downloaded file in a directory on the user's file system, you aren't going to be able to prevent it from being accessed by other applications (or the user) on the system. That's a security issue because that's how a lot of viruses work.
来源：https://stackoverflow.com/questions/998535/file-protection-in-adobe-air-flex