[...]

[...]

Works OK for me. You might examine the error stream to see where it's
getting lost. Also, you may need to use -a in your command line to
specify the application.

import java.io.*;
class ExecTest {

   public static void main (String[] args) {
       String s;
       try {
           Process p = Runtime.getRuntime().exec(
               "open /Documents/StudioDisplay.pdf");
           BufferedReader err = new BufferedReader (
               new InputStreamReader (p.getErrorStream()));
           while ((s = err.readLine()) != null) {
               System.out.println(s);
           }
       }
       catch (Exception e) {
           e.printStackTrace();
       }
   }
}

In general, I don't think you want to use the first effort (a single
long string), since Mac OS X supports directory and file names with
spaces in them.  I also don't think you need to use the MRJUtils package
-- and even if you do, I suspect that the MRJOSType would really need to
be a 4-character Mac OS "type code", which would probably mean it needs
the space at the end.

But here's something I've done recently that I happen to know for sure
works.  If the string you have to open is a regular URL (be it http,
ftp, etc.), it can remain intact without trouble.  If it's a file, then
use the Java File class call toURI() to create a "file:" URI from it.  
Your string array will then need to contain two entries.  The first will
contain "open", and the second will contain the string (URL or URI, with
no need for any quotes provided by you).  Passing that to the exec()
method will get the desired reaction.  The URL will open with your
system's default app for its type.  The file will open with whichever
application is designated.  Keeping in mind that not all files with the
same extension necessarily open with the same app, that could be Preview
or Adobe Reader, depending on your system and the selected file.

HTH.

= Steve =