This tripe does not deserve to be called a documentary. It is in fact akin to one of those stupid TV shows that show self-styled 'paranormal' investigators acting like children afraid of the dark.
For the real facts of this bogus case check out Wikipedia from which I quote.
"In his 1993 book Possessed: The True Story of an Exorcism, author Thomas B. Allen offered 'the consensus of today's experts' that 'Robbie was just a deeply disturbed boy, nothing supernatural about him'".
"According to author Mark Opsasnick, individuals connected to the incident were influenced by their own specializations:
'To psychiatrists, Rob Doe suffered from mental illness. To priests this was a case of demonic possession. To writers and film/video producers this was a great story to exploit for profit. Those involved saw what they were trained to see. Each purported to look at the facts but just the opposite was true — in actuality they manipulated the facts and emphasized information that fit their own agendas.' Opsasnik wrote that after he located and spoke with neighbors and childhood friends of the boy (most of whom he only referenced by initials) he concluded that 'the boy had been a very clever trickster, who had pulled pranks to frighten his mother and to fool children in the neighborhood'".
I advise anyone who may be taken in by this idiotic film to suspend their judgment until they've read the true facts about this which are easily available on the net. It is sensational films like this that perpetuate dangerous medieval beliefs. The makers should be ashamed of themselves but I'm sure they're too busy counting their money to worry about that.