Airmail allows you to save the emails as PDFs. This can be done from the message preview/reading pane. Please refer to the screenshot below to create a PDF.
We can create shortcuts manually to convert an email into PDF file. Please check the following screenshots:
Please go to Airmail preferences > Actions > Menu Buttons 1/2/3/4 and select PDF action with any of the Menu buttons:
The PDF button will appear at the top of the preview panel like this:
Similar to the Menu buttons, we can select the PDF action with Left or Right Swipe action.
Please go to Airmail preferences > Actions > Left/Right Swipe > PDF
You can swipe the desired email in the email list to perform PDF action:
Similarly we can set the PDF action to other shortcut options available in the Airmail preferences > Actions tab