Even though, and even as a self-confessed geek, I honestly can't remember how many years it is since I last needed to zip or unzip an archive on a phone, the availability of an archive utility on the platform does add a certain reassurance. Of course, an extra caveat is that, even under Windows Phone 8.1 and even with a microSD card available, you still don't get full access to the phone's file system. Still, for preparing a ZIP file for someone else, or for accessing an archive someone has sent you, 8 Zip might still just about fit the bill.
8 Zip bears all the hallmarks of a port from other platforms and codebases - the full feature/compression-formats and the rudimentary UI here - but it does work on the whole, albeit without any finesse. Though most of that is surely due to Windows Phone's own file and folder access routines being fairly basic as well.
Here's 8 Zip in action:
There's a choice of three graphical themes, a very nice touch...
There's no way that I can see to name your archive from within 8 Zip, so presumably you'd have to use a file manager too; (right) compressing multiple items (even folders) works as advertised, and quite quickly, too.
A very full range of formats and compression methods are supported, with (surprisingly) a choice of the full range of ZIP compression speeds (a tradeoff with eventual file size)
Working with an archive that has been received - in conjunction with a decent file manager under Windows Phone 8.1, this could be a powerful solution rivalling similar operations on Android.