The Collaborative International Dictionary of English v.0.48:

Viewer \View"er\, n.
   1. One who views or examines.
      [1913 Webster]

   2. (Law) A person appointed to inspect highways, fences, or
      the like, and to report upon the same.
      [1913 Webster]

   3. The superintendent of a coal mine. [Eng.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

viewer
    n 1: a close observer; someone who looks at something (such as
         an exhibition of some kind); "the spectators applauded the
         performance"; "television viewers"; "sky watchers
         discovered a new star" [syn: spectator, witness,
         viewer, watcher, looker]
    2: an optical device for viewing photographic transparencies

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "viewer":
   TV-viewer, accepter, acquirer, addressee, audience, auditor,
   beholder, bird-watcher, bystander, consignee, drugstore cowboy,
   eyewitness, gaper, gazer, gazer-on, getter, girl-watcher, goggler,
   hearer, holder, kibitzer, listener, looker, looker-on, observer,
   obtainer, ogler, onlooker, payee, perceiver, percipient, procurer,
   receiver, recipient, seer, sidewalk superintendent, spectator,
   spectatress, spectatrix, taker, televiewer, television-viewer,
   trustee, video-gazer, watcher, witness



The Free On-line Dictionary of Computing (30 December 2018):

viewer

    A program to allow a file to be read (or played) but
   not changed.  Viewers are often freely distributable, even
   when the editor application is not.  This allows you to create
   files with the editor and make the viewer available to other
   users to view your files, e.g. on a website.  Examples
   include the Word and Adobe Acrobat viewers.

   (1997-08-29)