How to cache a file e.g PDFDocument, to prevent re-loading and increase user experience - swift?

Question

I have an app with a collection view that displays thumbnails of pdf documents. When a user taps on the thumbnail a new UIViewController is presented along with the PDFDocument. I am trying to cache the PDFDocument so that the app does not have to download the PDFDocument from the server every time. I have not been successful. My code downloads the file every time from the server instead of accessing the cache. I am using PDFKit to present the PDFDocument. Below is my code:

When a user taps on a thumbnail in the collection view, the following UIViewController is pushed onto the stack and displays the PDFDocument:

 @IBOutlet weak var myPDFView: PDFView!
 let pdfCache = NSCache<NSString, PDFDocument>.init()

 func loadPDF(pdfURL: String) -> Void {

   // FIND IF PDF DOCUMENT IS ALREADY IN CACHE ON KEY 'pdfURL'
if let documentToCache = self.pdfCache.object(forKey: pdfURL as NSString){
    self.myPDFView.document = (documentToCache as! PDFDocument)
    self.myPDFView.displayMode = .singlePage
    self.myPDFView.autoScales = true
    print("PDF FROM CACHE!")
    return
}


 .....

// Not found in cache.  Download pdfDocument
Alamofire.request(pdfURL).responseData { response in

   if let data = response.data{

        DispatchQueue.main.async{

            let document = PDFDocument.init(data: data)

            // STORE PDF DOCUMENT TO CACHE
            self.pdfCache.setObject(document!, forKey: pdfURL as NSString)

            // DISPLAY PDF
            self.myPDFView.document = document
            self.myPDFView.displayMode = .singlePage
            self.myPDFView.autoScales = true
            print("PDF DOWNLOADED: \(pdfURL)")

        }
    }

}

}