Yesterday evening (25-Nov-2014, CET) I tried to export to PDF a book that I created and saved inside it.wikipedia.org, but it reported the following error:
<<Conversione non riuscita - La generazione del file del documento non è riuscita. - Stato: Rendering process died with non zero code: 1>>
I tried again 2 or 3 times, and I also tried to remove some pages, with the same error.
The same error occurred also when trying just to export a single page as a PDF.
I tried again today (around 15:00 CET 26-Nov-2014). I also tried just to "Download as a PDF" the Main Page of English Wikipedia.
The error message is now shorter:
<<Book rendering failed<br>There was an error while attempting to render your book.>>
For your guidance, I tried yesterday with Mozilla Firefox in a Linux Mint (Olivia) environment, and also with Android Browser from a smartphone.
Today I tried with Mozilla Firefox 33.1.1 in a Windows 7 environment.
The first error was reported by another user on the 9th of October (https://it.wikipedia.org/wiki/Discussioni_aiuto:Libri#Problemi_con_la_conversione) - maybe on other dates on other wikipedias.