You need to use a camera memory card for storage.
Those devices have mixed results depending on the application. If the application does not support the device, then you are out of luck. There is no file manager in IOS to allow you to move files between folders because there are no folders that you can access. An app has to specifically support a storage location to access files.
Why go to the trouble of copying the files to a card that camera supports, get the USB cable to connect to the camera, then hope? Just use iTunes. Connect the device to iTunes, click on the IOS device, click on Apps, scroll down on the iTunes main screen, where file sharing shows up, select the App, and then select Add File. You can transfer as many files as will fit in memory. Works every time, 100% of the time, and is free.In particular the Adobe Reader App supports this type of file transfer.