I've got the same problem. As it is not possible to update the pre-installed Documents to Go via Google Play (I always get error -104) I uninstalled that app using Titanium Backup. But unfortunately, it is still not possible to install ANY version of Documents to Go. If I try to sideload the APK file, I only get "App was not installed". Google Play gives me various errors, mostly -4 or -104, refusing to install the app, either.
In System/apps, there is no remnants of Documents to Go as far as I can see, a directory named cust does not exist.
Thank you very much in advance and kind regards,