Assuming you can bundle the data up, say via zip, you might want to look into using 'Share'...AirDrop, Messages, Mail, DropBox, iCloud - some of course involve servers, but when you say without, I figure you mean your own.
Actually, I mean without servers entirely. Assume iPads are distributed to end-users where they'll collect data, and then, later, returned to the researchers. One ideal solution would be for the researchers to connect each iPad to a Mac via a cable and transfer data across the cable to the Mac. As you are no doubt aware, XCode enables a developer to view connected iPad logs. I was hoping for something similar, but for local data access.
I've considered using the 'Share' mechanism, as you suggest, but was wondering if there are any other options.
>One ideal solution would be for the researchers to connect each iPad to a Mac via a cable and transfer data across the cable to the Mac
That won't work unless the logged-in owner on the mac, and the iPad, are the same itunes/apple ID user. You can't just casually cable an iPad to anyone's mac and browse it's contents/files like it's a thumbdrive.
Yes, you can connect via Xcode and use the Devices and Simulators menu to sniff logs, but that process doesn't allow for casual data browsing.
I think it would be much simpler to look into using AirDrop in your example. No cables, just Wi-Fi, and, when properly authorized, you can send to someone's else's nearby device, not just your own. See https://support.apple.com/en-us/HT203106
Other methods include 3rd party file sharing apps for iOS found in the App Store, and portable Wi-Fi drives.