you can be sure if Bill and Hillary could owe no income tax they would jump on it. As an employee I have taxes with held each week, end of year I get deductions from income, mort interest, Ira contribution, personal exemptions, property tax etc that reduces my taxable income. I get a refund because I had more tax with held than I owe based on taxable income. i stil pay taxes but get some back.
one expense many businesses have is depreciation, not tangible , but assets have a certain life period so if you own trucks, construction equipment, even printers you can depreciate these assets over a period of time that helps to reduce your income. if you are real estate agent you can deduct for mileage and other car expenses and on and on.
Any accounting firm worth it's cost will look for any and all deductions that legally apply to an individual or business. If his accountants followed the tax code there are no issues to have a tantrum about.
with the scrutiny he is under you can be sure the IRS. would go after him if something was amiss.
As the beast likes to say, "what difference does it make"
Trump said he was smart but its his accountants who are smart and know the tax code as it applies to Trumps businesses