Several types of external storage expansion are available for Apple iPad devices. When internal storage space on an iPad runs low, one of the simplest and best supported ways to free up space is to use cloud storage services such as Apple iCloud to move data into the cloud. Apple provide a significant amount of free storage with iCloud, but charge for large storage plans.
Like many companies in the tablet space, Apple use internal storage tiers as a method to sell devices at multiple price points. As a natural consequence of this strategy, they do not provide an in-built way to expand or increase the internal device storage capacity.
There are external hard drives that connect to the iPad directly through the 30-pin dock connector or Lightning cable on the device, such as the Leef iBridge or RAVPower FileHub. Various mobile companion devices with a similar form factors to external hard drives can be used as external storage for iPads. Some of these, like the Kingston Mobile Companion, accept secure digital cards.
For a long-term solution for archiving files and freeing up space, Apple's Time Capsule is compatible with the iPad. Another option is to invest in a personal network access storage device. NAS manufacturing companies such as Synology provide mobile apps that work on iPad to make the transfer and backup of files from the iPad to an NAS device simple.