Here's the relevant passage from Trump Jr.'s interview with the Pittsburgh Tribune-Review:
When asked why his father has not released his tax returns as presidential candidates have traditionally done, Trump Jr. said, "Because he's got a 12,000-page tax return that would create … financial auditors out of every person in the country asking questions that would detract from (his father's) main message."
That seems to me a remarkably honest answer from DJTJR (that's my preferred abbreviation for Donald Trump Jr). Donald Trump is a very wealthy man and a real estate developer. Everything we know about the tax returns of very wealthy people and developers is that those returns are very complicated and the principals almost always take advantage of every loophole available to them to pay less money. (Developers regularly report negative income for a year and, therefore, pay no taxes.)
No matter what's in there, you can be certain that Trump's tax return would look nothing like the tax return of the average person he is trying to persuade to vote for him. They would be exotic — in much the same way Mitt Romney's tax returns were in 2012. As I wrote about Romney's return in July 2012:
The tax returns speak to a broader Romney problem: convincing average people that he is like them or, at the very least, can understand their lives in some meaningful way.Romney’s background — his father was a governor and ran for president — is different than most people. His religion — roughly 2 percent of Americans are Mormon — is different than that of most people. And his wealth — as evidenced by his tax returns — is different than most people.
Trump has similar problems. Born in New York City into a wealthy family, Trump has achieved a level of wealth that the average person can't even comprehend. And when it comes to his tax returns, he himself has proudly admitted that he exploited every possible angle to pay as little in taxes as possible.
"I fight very hard to pay as little tax as possible," Trump told ABC's George Stephanopoulos in May. When Stephanopoulos asked what Trump's tax rate was, the real estate mogul responded: "It’s none of your business. You’ll see it when I release. But I fight very hard to pay as little tax as possible."
What Don Jr. did Wednesday then was confirm what we already knew: The reason Trump isn't releasing his tax returns isn't because he is being audited. (It's also not any of the myriad other reasons he has offered to explain why he isn't releasing them.) The reason is simple: He has calculated that the damage done to his campaign in releasing the returns is far worse than the negative press he gets from not releasing them. That's it.