Tax returns and work papers - Especially documentation used to carry over or carry back a deduction. Keeping all checks is recommended. Vouchers for payment to vendors, employees, and basically any and all documentation to justify the check or cash spent for a deduction. Records of inventories of products claimed at the end of the year. Payroll records, especially time sheets with all deductions and appropriate tax tables used.
Expense reports with documentation receipts.
An accountant will give specific advice on this matter. Purchase orders for any products or services sold, contracted for, or performed. Expired contracts and leases . Individual withholding tax statements for all employees. Subcontractors' bills and payment records.