usability – iOS download behaviour for pdf files in a shop

Sup guys, long time lurker first time poster here.

I developed a small shop for a client and have stumbled upon a bigger problem.

After completing a purchase for an e-book, customers can download it in the order overview. However, iOS does not load it into the file system but opens it in a browser window, which some customers probably overlook or click away so that they think that the shop is broke. I received some messages stating that they are not able to download it.

In addition, a limit is set for the downloads, which the customers can also see, but seem to use up because they spam the button.

What would be the best way to overcome this? I though of a modal stating that they should download it via the iOS download button afterwards, or a text block with the same message underneath the download button.

Any inputs are highly appreciated.

Thanks in advance!