Is it possible, on a Mac, to create a password-protected zip file, that can be decompressed on a PC and on Mac without problems? Is there a free way to do it if possible?
The terminal command
should do the trick. You will be prompted to enter and verify a password.
The encrypted file can be unzipped from the Finder, you'll be prompted for the password. The file should also be useable on Windows, 'nix OS's, and every other file system that supports zip:
source: zip man page
If you want a GUI solution, it shouldn't be too difficult to create an Automator workflow to automate this process.