Exactly as Michael said... but beyond in-app downloads, or dropbox sharing (you need a dropbox account and the dropbox iPhone/iPad app)...
Many apps, including BIMx and iVisit 3D, support document management via iTunes.
As seen in the attached screenshot, when your device is connected to iTunes (either by cable or, if IOS 5+, wifi), the bottom of the "Apps" tab for your device lists those apps that support file transfer.
In my screenshot, I've highlighted iVisit 3D, but BIMx is the same. You will see whatever documents are in that app's private storage and can delete them, drag them to your desktop (etc) - or can drag and drop new files into the Documents panel to transfer them into the private storage area for that app on your device.
This is also illustrated in detail in this Wiki article for BIMx:
http://www.archicadwiki.com/BIMx/iOStransfer
Anyone using BIMx really MUST read that Wiki article to fully understand the transfer options available.
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB