Crippin 1.4 uses RSA encryption to quickly and easily secure your files. "Crippin encrypted files are generally less than half the size of the originals, so it can be used for compression too. If you only want to use it for compression, just leave all the password fields blank."
It is available for all Pocket PCs and costs $7.95. There is a trial version available.
I can't claim to be impartial (I wrote Crippin, after all), but at least one of my customers spontaneously commented that he much preferred Crippin to F-Secure. I've never used F-Secure, so can't comment on what's wrong with it, but Crippin is very quick and easy to use. You can download a trial version from PocketPCCity: