On the Mac you can Preview any file by selecting the file and tapping on the Spacebar which will open the item in Quick Look. However, the Files App in iOS has not really had any type of Preview capability, until now that is. There is an easy way to Preview files in iOS 13, here is how that works.
Tap into the Files App on your iOS Device. The Files App looks like an ordinary Folder, but it really is an application:
Once in the Files App, find a file to work with. In my case I am going to work with a file in the Pages Folder:
Inside this folder I am selecting a sample Resume:
By tapping and holding on it iOS produces this menu with several choices:
Just tap on “Quick Look” and a Preview of the document will display on your iOS Device screen:
Yes, it is a little small on this iPhone 11, but you can pinch and stretch it to see a little more detail. It would be just fine on an iPad, but at least the Quick Look Preview gives you an idea what the document looks like.
Even though the Files App does not have a builtin Preview function, you can use some of the iOS 13 commands to Preview your files anyway.