David Hewson - I personally ascribe to the view that piracy is not your problem... obscurity is the problem.
How many people can actually read a PDF well - like a book? if it is 0,5% it is a LOT. The fact that these SPAMBots are using your work for it is more like a form of advertising (I had not hear of you to be honest)

I've downloaded (legally) a number of books... among them 'Free' by Chris Anderson - I also downloaded (Free) the audiobook and then bought the actual copy... and next time he comes to speak somewhere in NL I'm probably going to fork over E 100 or so to see him speak.  
The problem becomes when your actual product does not give anything more than the actual words.... try reading C@ntent by Cory Doctorw (@doctorow) and you'll get what I mean.

I think SPAM - any form... but especially on personal media such as a Phone or something like Twitter (which you admit to have done yourself !) is a far worse symptom. It is clogging up our ability to use a system (the internet) well.... so get as mad at yourself for doing just that! 
Call it Karma that you now are getting worked up over this!