Android DownloadManager Class: Don't Want to Download pdf files Multiple Times in Android Studio

As Shown the image like that I want in Android studio for pdf Files

File file=new File(Environment.DIRECTORY_DOWNLOADS,filename);
if(file.exists){
// do something
}
else{
// do something
}`

But the Above Code is not Working and it creates a new file..What is the code for Android Studio..Please Help me.