Error: File does not begin with '%PDF-'

instead of opening the pdf when clicking on the link - save the link as a file. open this file using notepad or vi and see what shows up. You may be seeing messages generated by the TCPDF class that get output before the PDF is streamed.

Thanks, I'll look at that. I think I may have fixed it though. Turns out it was my fault. I have modified this module so that books don't start a new page for each node and I had commented out one line in one of the functions. I have now wrapped that line in an if statement which bypasses it only for books. Things are fine.

I see I'll probably have to do more work on this module over time. If I make any significant upgrades, I'll post them for all to use. And now you've given me a good testing technique, so thanks.