Download Issues with iOS (iPad, iPod, iPhone)
Prior to iOS 13 devices like the iPad, iPhone, and iPod are fundamentally unable to download and store files from the web. They will try very hard to display them, however. As an example, PDFs will display in Safari just fine, but you can't save them from there, you can only view them.
As of iOS 13, iOS devices have the ability to download certain files. Rather than viewing or streaming from the browser, a user sees a prompt to Download the file. This downloads to the device's iCloud Drive.
For different behavior, you can use Amazon S3. In most cases if your file is hosted on S3, if iOS supports the file type, it will be accessible via the browser. For example, a mp3 can be played via the browser rather than downloading it.
For other types of files it will try to open the proper application to view those files. If it can't find one it will ask what application should be used. See screenshot below:
If you or your customer need the file on your iOS device we recommend it be downloaded on a computer first, and then use iTunes to transfer the file to the device. Instruct your customers to open the email receipt from their main computer, and transfer the file to their device using iTunes.