Why is Adobe Acrobat Send Mail not working?
We’ve researched the email integration between Adobe Acrobat Send Email and email apps. From our testing on Mac, the list of apps that Adobe supports sending PDFs to is limited to Apple Mail and Outlook. For instance, Adobe does not work with Mozilla Thunderbird or Airmail, which are two very popular email apps for Mac. Unfortunately, this leaves little hope for an Adobe / Missive integration anytime soon.
You will need to save PDFs as files, then include them in your Missive drafts like other attachments.