I am once again asking for a new feature. I just discovered that there is no way to save messages from K-9 (soon-to-be Thunderbird for Android) as PDF that can be then attached to another e-mail. I can export it using system share feature (which by the way has terrible name in K-9 – in Polish it's just "Forward (alternative)"), but it only shares it as txt, which doesn't look great or fully preserve formatting, for example text color (not to mention, I then have to export is as txt again to save it on my phone and add as another mail attachment).
While most people probably won't find this feature very useful, it will bring mobile Thunderbird a step closer to feature parity with desktop version and add another benefit for people considering switching from Gmail, Outlook and other mobile email clients.