Airmail can integrate with Google Drive. To enable the Google Drive integration, please go to Airmail Preferences > Services and then 'Link Google Drive' as shown in the picture below.
You can save the files on Google Drive (as .pdf files) and attach the files from Google Drive in the Airmail composer.
How to save a message to Google Drive from Airmail?
Please follow these steps.
1. Right click on a message and select 'Send to' or just press the 'Z' shortcut key to open up the share menu.
2. Then choose 'Save to Google Drive' as shown in the picture below. You can also configure 'Save to Google Drive' as a 'Menu Button'.
3. Then you will find the following dialogue window and in this window, choose the subsequent path.
How to attach using Google Drive?
You can attach using Google Drive in Airmail Composer as shown in the picture below:
Once attached and the upload finishes, a link will be created on your linked Google Drive account as shown in the picture below: