Its a purposeful choice. This way PP gets to scream about government secrets to his sycophants. If he went and did the right thing, he would both be properly informed and unable to constantly spew conspiracies.

He should already have a security clearance working in government. Not being able to get one or not being willing to get one should disqualify you from office.

Because, as Bloc Quebecois Leader Yves-Francois Blanchet puts it, it’s a “dumb trap.”

Just a hunch, but I'd imagine that one of the conditions of getting a security clearance is "don't tell anyone what we're about to show you" and he won't promise not to expose anything that makes the liberals look bad.

Well if we're speculating, I'd guess plausible deniability. Knowledge is power.

For example, say there's something in the docs he already knows about (in broad strokes) but he needs an excuse for not being prepared to handle it once he's in office (i.e. anything with negative outcomes that can only be averted with direct public sector spending), OR he has a source willing to leak documents to him and wants plausible deniability so that when something that he feels he could spin his party's way gets leaked to media to manipulate opinions, it won't be traced back to him.

EDIT I want to add that I don't think only the CPC exclusively would do this stuff, but I've heard reports journalists need to be wary of intentional leaks for political gain and I'd be very surprised if the CPC wasn't participating in this kind of thing.