Airmail now introduces a new feature 'Save to iCloud Drive' which allows you to save your emails (in PDF form) and attachments to your iCloud Drive. Please note: iCloud Drive should be enabled to use this feature. Here is the article on 'how to enable iCloud Drive'.
This feature is now available in the 'Action List' and can be easily accessed as described below.
Saving on iCloud Drive while reading -
While reading a message, if you wish to save a message to the iCloud Drive then tap on the 'three dots' icon on the top right corner, then select 'Save to iCloud Drive' from the action list as shown in the picture below.
Saving on iCloud Drive using swipe actions -
You can also invoke the 'Action List' using a swipe action (left/right), right in the message list as shown in the picture below.
Then choose 'Save to iCloud Drive' from there. Please refer to the picture below for invoking the 'Action List' using a swipe action.
PLEASE NOTE - If you save the emails to the iCloud drive very often, it is advisable to configure 'Save to iCloud Drive' option in swipe actions.
To do so, please refer the article which explains 'Swipe Actions'.