IonicNative File writeExistingFile not working?

It gives me success on this WriteFile API and when I open the file of test.pdf after this function success it gives me error of Invalid Format.

Any who can help me out in this
Here is my code

 this.file.writeFile(cordova.file.externalDataDirectory,"Test.pdf","Hello World",true).then(succ=>{
      console.log("File write success : ", succ)
    },
    err=>{
      console.log(" write File error : ", err)
    })