Illustrator - Let the script only apply to one document

Question

I found such a script. But it works on all open files and takes a lot of time. How do I do it to only one current tab.

var doc = activeDocument; 
for(var i = 0; i < app.documents.length; i++)  
    for(var j = 0; j < app.documents[i].textFrames.length; j++){    
        var str = app.documents[i].textFrames[j].contents;    
        var rep = str.replace ("PANTONE", "P.");
        app.documents[i].textFrames[j].contents = rep;
          }
      redraw();

Answer 1

@joojaa your answer is too short. I don't quite understand but I found my own answer by trial and error.

for(var j = 0; j < app.documents[0].textFrames.length; j++)
{ var str = app.documents[0].textFrames[j].contents;
         var rep = str.replace("PANTONE", "P.");
         app.documents[0].textFrames[j].contents = rep; }