He pulled so much information, that it's likely he didn't know everything he pulled. James Clapper estimates around 1.5 million documents. Even if I had all year I couldn't read that. You'd have to read over 4100 pages per day every day for a year to get it done.
Ryan Gallagher says this about the claim that Snowden took 1.5 million documents: Although 1.5 million documents is a slightly different number, I wouldn't rush to believe James Clapper's claim here. Besides, he doesn't have the best track record for honesty.This 1.7m figure was invented by US officials and since then it has been regurgitated repeatedly and unquestioningly by various media outlets. I've seen the trove of documents; the claim or insinuation that he leaked 1.7m is not true.