Recommend File Format?

[highflight1985]

I'm designing an app for PDAs that relies heavily on user inputted data that is saved and retrieved often. What means should I use to save and retrieve the data? Here are some points to consider:

Requirements:
1. File needs to be transportable; that is, the user is able to share the file with other users.
2. File needs to be secure in that if the file were opened with a text editor, the user couldn't easily edit what was contained in it. It shouldn't plain text--it should be somewhat binary or cryptic.
3. Loading and reading of the file needs to be as fast as possible. Ten seconds to load a file that is a maximum of 10KB isn't acceptable, but 1 to 2 seconds is.
4. Need to be able to access the data within the file in a manner as close as possible to the way data is accessed from a database.
5. Needs to be easily upgradeable as new versions of my application are released--needs to not break the application, if in a new version, a new bit of data or a new feature is added, such as a boolean toggle to automatically update time/date.

Possible options I know of:
1. Database
2. XML (would have to be encrypted and decrypted on the fly, slowing things down a bit)

If a database is used, can the file extension be changed and still be treated by my application as a database?

I'd appreciate any advice anyone can offer.