One of the most useful apps, in my opinion, is FileBrowser by Stratospherix. I think that it will transfer any file from my Windows 10 PC and my NAS to and from my iPad and iPhone. There is, though, the big question as to whether anything will be able to read/ view/ play it? Sometimes you have to search a bit - or ask here, as to what would access a file with extension .xyz, if at all.
I would also suggest looking at Documents by Readdle. This will also transfer files between the different platforms.
We all have our favourite apps, but the two above would, I expect, be high on the list of most users recommendations if not on their iPads/iPhones.