Potential Trojan Horse download on iPad and iPhone 11

Due to the system architecture of iOS/iPadOS, unless jailbroken, your iPad is not susceptible to traditional malware infection per-se. However, as with all computer systems, there are still vulnerabilities and exploits to which you remain vulnerable. 

Be wary of the myth that Apple devices are immune to malware; those that perpetuate this untruth do not fully comprehend the broader threat landscape. Consider that if the myth (and over-generalisation) were true, Apple would not expend considerable resources as they do in developing and issuing regular security updates and patches for their products.

While your iPad is unlikely to be directly infected by malware, it remains possible to easily download an “infected” file to the iPad. While the malware payload is unlikely to run and won't be able to escape the App sandbox in which the infected file resides, if the file is transferred to another "vulnerable" computer, the file still has the capacity to infect other computer systems with malware.

As such, simply deleting the suspect file from your iPad will eliminate the threat from your iPad. A precautionary restart of your iPad will ensure that any executable code is purged from the running OS.

Force restart iPad - Apple Support

Its close to impossible for iPad or iPhone to be infected with a Trojan. The sandbox nature of iPadOS and iOS prevents anything from running outside of the area of the App it is in. So for starters it would not have been able to get out of the Mail app's sandboxed area. Second, if this was trojan meant for Windows, it would be entirely incapable of executing itself on iPadOS or iOS. Different platforms.

You can delete the email and that should be the end of it.

You're welcome.