The stonewall_lines.pgn file I downloaded is just a text file that is only 360 bytes in size. I also have a zip file for that (which may have been how it downloaded, don’t remember). The equivalent file for the Owens defense is 914 bytes in size.

I don’t have an iPad, but the problem may be that iOS doesn’t know what to do with a pgn file so it doesn’t have a default app assigned to that extension. Those files can be opened in most chess gui apps or uploaded into Lichess as shown by Nelson in the bonus content.

Looking at https://stockfishchess.org/download/, apparently there is a “SmallFish” chess application in the app store for iOS that is recommended for running stockfish locally on iOS. I would assume that application would be able to open pgn files, but haven’t tried it.

Hope this helps somewhat.