This mapping doesn't make any sense. To which view should PrettyFaces forward if the client requests /viewer
?
However, you can do something similar with Rewrite, which is the successor of PrettyFaces. With Rewrite you can do something like:
.addRule( Join.path("/viewer/{page}").to("/pages/{page}.xhtml") )
This will basically map your URLs like this:
/viewer/foo -> /pages/foo.xhtml
/viewer/bar -> /pages/bar.xhtml
/viewer/whatever -> /pages/whatever.xhtml
If you want to migrate your app to Rewrite, which is really simple, have a look at the PrettyFaces Migration Guide.