UIDocumentInteractionController download file failed when the PDF size is big - ios

Question

I download and display PDF from my swift application using this code:
var docController:UIDocumentInteractionController!
let pdfUrl = NSURL(string: "ENTER_URL_OF_PDF")
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
downloadDoc(pdfUrl: pdfUrl!)
}
#IBAction func buttonAction(_ sender: AnyObject) {
docController.presentOptionsMenu(from: self.view.frame, in: self.view, animated: true)
}
func downloadDoc(pdfUrl : NSURL) {
let urlTest = self.pdfUrl!.absoluteString
let pdfUrl = NSURL(string: urlTest!)
if(pdfUrl != nil){
let pdfRequest: NSURLRequest = NSURLRequest(url: pdfUrl! as URL)
NSURLConnection.sendAsynchronousRequest(pdfRequest as URLRequest, queue: OperationQueue.main) {(response, data, error) in
let httpResponse = response as? HTTPURLResponse
if(httpResponse?.statusCode == 200 && error == nil){
let documentsUrl = FileManager.default.urls(for: FileManager.SearchPathDirectory.documentDirectory, in: FileManager.SearchPathDomainMask.userDomainMask).first as! NSURL
if let fileName = self.pdfUrl!.lastPathComponent {
let destinationUrl = documentsUrl.appendingPathComponent(fileName)
if let data = data {
do {
try data.write(to: destinationUrl!, options: .atomic)
} catch {
print(error)
}
self.docController = UIDocumentInteractionController(url: destinationUrl!)
}
}
}
}
}
}
The problem I'm facing is that when I try with a PDF containing grater than 3 pages, I got this error:
fatal error: unexpectedly found nil while unwrapping an Optional value
and my app crash. When I searched about the max size that can be downloaded I read in man SO posts that there is no limit!
So why I got this problem?