Airmail can integrate with Dropbox. To enable the Dropbox integration, please go to Airmail Preferences > Services and then 'Link Dropbox' as shown in the picture below.
You can save the files on Dropbox (as .pdf files) and attach the files from Dropbox in the Airmail composer.
How to save a message to Dropbox 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 Dropbox' as shown in the picture below. You can also configure 'Save to Dropbox' 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 Dropbox?
You can attach using Dropbox in Airmail Composer as shown in the picture below:
Once attached and the upload finishes, a link will be created on your linked Dropbox account as shown in the picture below: