For Linux and Windows there are two viewers. One is a very basic viewer using x11 and win32, respectively. It has been supplanted by a newer viewer using OpenGL for rendering, which has more features such as table of contents, unicode search, etc. We keep the old viewers around for older systems where OpenGL is not available.

Read the manual for the new Linux and Windows viewer.

Android currently has several different viewers with varying degrees of complexity:

MuPDF
  The main app on Google Play. Supports forms and annotations.
MuPDF viewer
  A slimmed down viewer only variant of the main app.
MuPDF mini
  A minimal code example of a document viewer.

The iOS viewer is available on the App Store.

There is also a WASM viewer for running MuPDF in the browser.