35 ILCS 120/8
For the purpose of administering and enforcing the provisions of this Act, the Department, or any officer or employee of the Department designated, in writing, by the Director thereof, may hold investigations and hearings not otherwise delegated to the Illinois Independent Tax Tribunal concerning any matters covered by this Act and may examine any books, papers, records or memoranda bearing upon the sales of tangible personal property or services of any such person, and may require the attendance of such person or any officer or employee of such person, or of any person having knowledge of such business, and may take testimony and require proof for its information. In the conduct of any investigation or hearing, neither the Department nor any officer or employee thereof shall be bound by the technical rules of evidence, and no informality in any proceeding, or in the manner of taking testimony, shall invalidate any order, decision, rule or regulation made or approved or confirmed by the Department. The Director of Revenue, or any officer or employee of the Department authorized by the Director thereof, shall have power to administer oaths to such persons. The books, papers, records and memoranda of the Department, or parts thereof, may be proved in any hearing, investigation, or legal proceeding by a reproduced copy thereof under the certificate of the Director of Revenue. Such reproduced copy shall, without further proof, be admitted into evidence before the Department or in any legal proceeding.
35 ILCS 120/8