previewer

noun

1. A person who sees a film or TV program, etc. before its general release to the public.
2. A program that provides a preview of a document or other work.

   “For example, the Macintosh versions of TeX provide a screen previewer as a part of the basic package, whereas the screen previewer for the IBM PC is an extra.”

   “To view a file for which there is no previewer installed, save the file to disk and open it with the appropriate application.”