Lamination, according to some very specific and tested standards, used to be a favored procedure for preservation of valued documents. Lamination, when we think of it, involved a heating procedure that fused two plastic sheets in a sandwich around a document, with or without pre-treatment with alkaline compounds to absorb acids. However, lots of different products became available, standards fell over time, or operators were poorly trained in the evaluation and suitability of materials for this treatment. Another process used to mount and display photographs or posters called "dry mounting" may have been applied to documents as well. Dry mounting involves placing an dry, meltable adhesive between the object and a support board. This process is not a sandwich, but also is a heat and pressure process to ensure an adhesive bond. Lastly, this could have also been done with a cold-tack, pressure-sensitive adhesive, or spray-mount rubber adhesive. Some adhesives are more stable than others, and the quality of the mounting support can affect the long-term aging of the adhesive and the document.

It's hard to tell without looking at it whether you have a truly laminated document or a dry-mounted one. I'm guessing based on the date that it would be the latter. Do you have any other clues, such as a record of treatment, invoice, or a sticker on the back of the frame that indicates the framing shop and/or process?

Happily, mounting processes are somewhat reversible, but it is usually a labor intensive treatment which involves testing for immersion in solvents or application of solvent gels, although sometimes it is possible to split a dry mount bond just using heat and mechanical scraping. A paper conservator could tell you more and give an estimate of not only the amount of time and expense, but also the expected outcome. After removal, it might be desirable or necessary to wash and deacidify the document, remove stains from the adhesive, and/or provide a new suitable housing for it. Note that some of the characteristics of the paper, such as texture, or color may have irreversibly changed through the previous treatment and exposure, but some improvement can be expected. For alternatives to lamination for storage of documents, see our post on Lamination.