Before Acrobat XP, a PDF was essentially a flat sheet of glass. You couldn't turn layers on or off. Acrobat XP allowed creators to define layers within a PDF. Suddenly, an architect could send a single PDF of a building plan where the client could toggle the electrical wiring, plumbing, and furniture layers on and off. This moved the PDF from being a static "picture of a document" to a dynamic data container.
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