A proprietary viewer is a software package that makes information available to someone, either literally by displaying it on-screen for “viewing,” or more generally by making it accessible in whatever way is appropriate to the contents.
The notion that a viewer would be “proprietary” is a bit of a misnomer. The term itself suggests something about ownership, which is perhaps loosely true; but more specifically what is meant by the term is that the viewer program is a special program that “knows about” rights management systems. Such a viewer could, for example, understand that one user may have paid for the right to read the contents of a digital object, but not enough for the right to print those contents on paper; or paid for the right to watch a movie two times, but not three times. The viewer program would enable display of the contents of the object on a computer screen in the first instance, but not allow printing; or in the second instance, allow two viewings, but not three.
The point of such viewers is that a digital object can be made available to a buyer, without the contents ever being fully “in the clear.” That is, there is never a time when the contents are unencrypted and residing as a computer file of information accessible to the buyer or anyone else. Display or other access to the digital object’s contents always takes place under the control of the viewer program, and that program is designed to allow the buyer to do only what the buyer has contracted to be allowed to do.
Like any scheme of technological protection, digital objects and proprietary viewers are not insurmountable restrictions. They are based on computer programs; such programs can sometimes be “cracked” or “broken” or “reverse engineered.” When that is done, the information is no longer protected against unauthorized copying. But protection schemes will make such efforts harder than otherwise in time, trouble and especially in expertise.