I was about to go in to something I couldn't blow off, and I couldn't blow off returning the mail as soon as possible either.
But I had a Note 4 phone and a working cell connection and managed to get it all done in a few minutes right there in my drivers seat.
I think I didn't even end up using the Note4's stylus but writing the signature on paper and taking a pic.
It was a while ago so I don't remeber exactly what I did or what apps I used. I probably tried a few different things on the spot before I found something that worked. Maybe the official adobe android app had signature support like the desktop app does now, or maybe I had to do it more manually with some other combination of apps, but I know that I went from receiving an email with a pdf attachment to returning that email with my own new pdf attachment all in about 15 minutes with nothing but my phone.
I have a 600dpi png file with black foreground and transparent background.
Xournal drops it anywhere right over the existing pdf image, resized and positioned at will, and the transparent parts are transparent.
Now I just paste that over the appropriate place on electronic documents and email it back.