I have no problem using Google Drive to store files on my SD card (actually I have a Galaxy Nexus, so it stores it on phone as a virtual SD card). this works for zip files, google doc files, and several others I tried.
Firstly, you need to turn off Enable Encryption (encrypt offline documents) in the settings of the Google Drive app. Then find the file in your "my drive", select it, and make it available offline. This will cause Google Drive to download it to your phone.
As Supaiku mentioned, it saves it to a crazy folder located at /mnt/sdcard/Android/Data/com.google.android.apps.docs/files/pinned_docs_Files_do_not_edit/*CRAZY FOLDER NUMBER*. You have to use a file explorer (I use root explorer) to navigate there to see your files.
Hope this helps.