There are many ways to get a file onto and Android device, but the simplest ones require they be on another Android first. Following are some easy ways to do it without another Android.
If you are using the K-9 mail app and ES File Explorer, this is fairly simple.
Use an OTC cable to plug a USB Thumbdrive into your Android. OTC cables plug into the power charging slot and, on most modern Androids, allow you to plug in most USB devices (mouse, keyboard, thumbdrive).
You can then use the USB drive with any file explorer.
ES File Explorer can connect to workstations over SMB (Windows File Sharing) and FTP. Simply choose the LAN option, log into your workstation, copy the file, then paste it to your Android.
From your workstation, place the key on your OwnCloud account. Use the Owncloud client to download it.
The file will end up in your sdcard/owncloud/youraccount/ folder