using 7zip for several reasons ...
i) Agora doesn't like trying to send over a hundred PMs
ii) wherever sensitive material is concerned, it's the de facto simple archiver*
don't want to be a pain, but I do recommend installing it, especially since it can do all the other formats too (zip rar tgz etc)
GLTA,
R.
* - cross-platform (windows/mac/linux/etc), 256-bit AES, handles all the other formats, etc.
oh, and it's free. can't beat free.