You can write a text file to the Server's Filesystem using a Java Step like this:

try {
  PRFile prFile=new PRFile( "StaticContent/global/ServiceExport/test.txt" );
  PROutputStream pros=new PROutputStream( prFile );
  java.io.PrintWriter pw=new java.io.PrintWriter( pros );
  pw.println("hello");
  pw.close();
  pros.close();
  
}
catch(Exception e) { throw new PRRuntimeException(e); }

//

And to read one in ; (and say) Stream it back to the Browser, you can use:

try {
  PRFile prFile=new PRFile( "/StaticContent/global/ServiceExport/test.txt" );
  PRInputStream pris=new PRInputStream( prFile );
  byte[] bytes = org.apache.commons.io.IOUtils.toByteArray( pris );
  tools.sendFile( bytes, "test.txt", false, null, false );
}
catch(Exception e) { throw new PRRuntimeException(e); }