You might want to check the VAT position. So far as I can make out most PDF's do not attract VAT in the UK. It is complex but if it is less than 50% advertising it seems to be okay. Amazon have stopped charging it on their ebooks. Gives you a bit more money- prices did not fall when VAT stopped!
Incidentally I fully agree it is very complicated! You have different rates in different countries and different things are subject to VAT. I got the impression the idea was to confuse everyone - and they certainly have done that!
Roger