I'm embarrassed to say that I can't find much online documentation on 
smalltalk projects and their ".pr" file representation.  But the quick 
answers to your questions are:
   ".pr" is the extension for a file containing a Squeak project.
   Squeak is the program that interprets it.

The following web page has simple instructions on how to load a project 
from Bob's SuperSwiki:

    http://minnow.cc.gatech.edu/squeak/1875

The minimally functional Scamper browser _should_ offer a "do you want 
to load as a project" dialog when you click on a ".pr" URL.  The world 
menu and the FileList also provide ways to load projects.

Richard

> Date: Mon, 20 Oct 2003 17:12:13 +0200
> From: Trygve Reenskaug <trygver at ifi.uio.no>
> To: The general-purpose Squeak developers list
> 	<squeak-dev at lists.squeakfoundation.org>
> Subject: MorphLayoutArticle
> Message-ID: <5.2.0.9.2.20031020165415.00bb36b8 at uio-pop.uio.no>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> MIME-Version: 1.0
> Precedence: list
> Reply-To: The general-purpose Squeak developers list
> 	<squeak-dev at lists.squeakfoundation.org>
> Message: 27
>
> I'm getting into morphic and I like what I see.
>
> http://minnow.cc.gatech.edu/squeak/2141 says
> "Please read the excellent dynamic essay project MorphLayoutArticle on
> Bob's SuperSwiki.
>
> I go there and find MorphLayoutArticle.018.pr.
>
> What's a .pr-file? Where can I find a program that interprets it? I've
> tried decoding it using Scamper. It believes that it may be a project 
> - but
> fails when trying to decode it.
>
> thanks
> --Trygve