OK, tried a lot more different PDFs, none work.
Thanks for your advice babyd, I did a hard reset, copied just the Sony manual via the Sony software, then used the menu to turn it off.
So when I turn on the device I have only the Sony manual, I go to 'books by title' and see the PDF, but when I press the '1' button to select the PDF the reader reboots.
I did take it to a local Sony shop as I work in central London, but they claim the online Sony is a different company so they could not replace my reader, but they gave me the customer support phone number.
So I have a support reference number with Sony now they are supposed to call me back with some help, I will update when something happens.