Files that are saved in the ‘File’ tab of ‘Send’ menu can be opened up with a separate app.

Choose a file and tap ios_more_bottom button then tap Open in. Choose the app you want. If you can’t see the app you are looking for, tap ‘More’ on the far right of the list. Then enable the app.

You can also go to the preview page and use share_blue_100 in left bottom corner.

Very rarely, ‘Open in’ function might be disabled depending on the file types.