If you have local access to the VPN server, then you should download the key directly from the server to each device you eventually want to connect remotely. This is the easiest and most secure way to pass keys. For example, I use pfSense, so I just open the psSense GUI on my device (phone, tablet, computer, etc), browse to the VPN section, and download the key directly to my device. I can then use that key during the setup of the VPN connection without having to email the key or upload it to a cloud server.
Just be aware that most email and online storage is not encrypted and therefore your key could be exposed. Now I will admit that the odds of someone getting an encryption key AND having the knowledge to be able to access your VPN connection is pretty small. But if you have passed that information along via the same unsecure method (a different email for example), then it might not be such long odds.