Useful file locker
It's a file locker. You load files into it and it shows you and lets you manage those files that you've loaded into it. It lets you upload/download via a built-in FTP server, a built-in web interface or WebDAV, so it's relatively easy to load files in that you want to take with you. It also lets you share those files with other applications, including copying files back and forth with the stock Files app.
I keep a slew of user manuals for all kinds of gear on it, as well as other PDFs and some video files; and FTP works pretty well for transferring large files to/from it in general.