how to declare long size variable in nodejs

Question

I am trying to assign base 64 string of big pdf in javascript variable but its not assign complete string properly.so what I do for assign complete string in javascript variable in nodejs.My code is:

 var child = exec('java -jar .....some variables,
                    function (error,stdout,stderr) {
  // stdout recieved base 64 pdf string
  // pdf size is aprox 15 mb so string also large
        console.log('Output value -> ' + stdout);
                            var data = {
                                base64PdfString: stdout
                            } 
    })

so when I try execute this function Its return large base64 string in my callback variable but when want to use this variable than string is not recieved completely so pdf is not create properly.its create some error in pdf.thanks.