I was going to type it out, but I found this at another forum:
The papers the brothers had payed to get were reports on the Orange crop turnout for the season. The more oranges produced, the more OJ can be made, which lowers the value of a can of frozen OJ on the market. The report would have told them how much OJ was going to be worth once the report was made public, so they would be able to buy up all the stock before hand, driving up the price before selling it all at a certain price. Since Dan & Eddie switched the reports, they were going on reversed info, so they thought they'd be able to buy more stock longer than in reality (thus, they would be able to sell when it got to a certain number and laugh all the way to the bank.) When the Head of Commerce reads the real report, they find that the orange crop did better than they thought, so the price plummeted instantaneously. Since they had snatched up such a large portion of stock, they couldn't sell their trades fast enought to cover the loss of the price dropping from tens of dollars to just a few cents per trade. Since Dan & Eddie sold all their stocks at the peak price (because they knew), they became millionares while the brothers were reduced to rubble (until Eddie makes a generous donation to them in Coming to America.)