If you are having issues with loading the TLS certificate from Burp Suite Pro onto your iPhone/iDevice running Apple's iOS 12 you are not alone.

It seems that when you click on 'CA Certificate' after browsing to http://burp it now only downloads the .DER file, instead of asking to load it as a Profile.

The one solution I found is as follows. You can load it from the Mail app found on your iPhone/ iDevice. Send it via email to yourself and load the email via the Mail app. From there it will prompt you and install it correctly. Just don't forget the next step.

Don't forget to go to the "Enable full trust for root certificates" as detailed here: https://support.apple.com/en-nz/HT204477. Otherwise you might still have issues getting secure traffic through.

Happy proxying.

Test setup:
Burp Suite Pro v1.7
iPhone iOS 12