Evidence in a contested case, including records and documents in possession of an agency of which it desires to avail itself, shall be offered and made a part of the record. Other factual information or evidence shall not be considered in determination of the case, except as permitted under section 77. Documentary evidence may be received in the form of a copy or excerpt, if the original is not readily available, or may be incorporated by reference, if the materials so incorporated are available for examination by the parties. Upon timely request, a party shall be given an opportunity to compare the copy with the original when available.