You’re holding it wrong. Or rather, that is not the way Ipads handle files.
On your PC, you get sent a file, then you can save it in your Documents folder and open it with Word, Excel, or whatever. Perhaps you use Excel to create a graph, save it in your Documents folder, then add it to a Word document by looking in your Documents folder and importing the file into Word.
That is not how Ipads and IOS/IpadOS work or handle files. You need to get away from that kind of thinking, as it will just make things difficult for you.
Each application on the Ipad has its own file area. Some of those file areas are exposed in the Files app, some are not. If you go into Files you may see folders associated with a few of your apps. If you use Files to put something in one of the app specific folders, then that app will be able to see it.
So, you probably put your documents into the Verbatim app’s Documents folder, not some device wide Documents folder.
Usually the way to pass a file between apps on the Ipad is by sharing. In the Verbatim app, or other apps, you may be able to tap or long tap on the file, and then “share” it. That should give you a list of apps it can be shared to, and will then be visible in or opened with.
That is how I get movies from off my Ipad into VLC (a media player) on my Ipad. I download or copy the movie into my equivalent of the Verbatim app, and then share the movie with VLC. That copies the movie into VLC’s file space, and then I can play the movie. At that point I can also delete the copy in my equivalent of the Verbatim app.
Cloud stuff can often make this much easier, as the Microsoft and Google office programs make it easy to open files stored on their respective cloud services.
I am definitely not saying the way the Ipad does things is better, or even really makes any sense; just that’s how it is.