I was recently checking for new iOS file managers since I discovered the one I've been using is no longer on the App Store. It still works fine but I'm concerned about ongoing support.
I found this one which was updated only a few days ago so is still actively supported. It allows me to connect to Caustic server and cloud storage for backups and file management. And it's free.
Documents 5 - Fast PDF reader and cloud download manager by Readdle