Modifying a PDF in iOS

Question

I am trying to modify a pdf. This program Creates a simple PDF when ViewDoesLoad. That works. Then, when the button is pressed, I reload the PDF, and try and write additional text to it and then save it. However the the modified PDF is Identical to the original? What did I do wrong?

BTW I added the whole program so that it is easy to insert in a simple stupid sample app. I need to write more details here for StackOverflow to allow me to post. So you can skip this paragraph.

import UIKit
import PDFKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        let pdf = createFlyer()

        if let docDir = documentsDirectoryPath() {
            print (docDir + "/newPDF.pdf")
            saveDataToFile(data: pdf, filePath: docDir + "/newPDF.pdf")
        }
    }

    @IBAction func button(_ sender: Any) {
        print("pressed")
        if let docDir = documentsDirectoryPath() {
            print (docDir + "/newPDF.pdf")
            let inputURL = docDir + "/newPDF.pdf"
            let outputURL = docDir + "/modPDF.pdf"
            if let input = URL(string: "file://\(inputURL)") {
                // Use the URL object
                if let output = URL(string: "file://\(outputURL)") {
                    // Use the URL object
                    modifyFlier(srcURL: input, dstURL: output, text: "new string")
                }
            }
        }
    }
    
    //https://stackoverflow.com/questions/64376645/swift-ios-overlay-text-onto-pdf-with-pdfkit-and-ui
    func modifyFlier(srcURL: URL, dstURL: URL, text: String) {
        // Confirm there is a document there
        
            if let doc: PDFDocument = PDFDocument(url: srcURL) {
                
                // Create a document, get the first page, and set the size of the page
                let page: PDFPage = doc.page(at: 0)!
                var mediaBox: CGRect = page.bounds(for: .mediaBox)
            
                // This is where the magic happens.  Create the drawing context on the PDF
                let context = CGContext(dstURL as CFURL, mediaBox: &mediaBox, nil)

                UIGraphicsPushContext(context!)
                context!.beginPDFPage(nil)

                // Draws the PDF into the context
                page.draw(with: .mediaBox, to: context!)
                
                let flipVertical: CGAffineTransform = CGAffineTransform(a: 1, b: 0, c: 0, d: -1, tx: 0, ty: mediaBox.size.height)
                context!.concatenate(flipVertical)

                let attributes = [
                    NSAttributedString.Key.font: UIFont.boldSystemFont(ofSize: 72)
                ]
                let text = "I'm a PDF!"
                text.draw(at: CGPoint(x: 0, y: 0), withAttributes: attributes)
                
                context!.endPDFPage()
                context?.closePDF()

                UIGraphicsPopContext()
                
                if doc.write(to: dstURL) {
                    print("Modified PDF saved successfully")
                } else {
                    print("Failed to save the modified PDF")
                }
                
                print("The pdf should be different")
            }

    }
    
    func createFlyer() -> Data {
      // 1
      let pdfMetaData = [
        kCGPDFContextCreator: "Flyer Builder",
        kCGPDFContextAuthor: "raywenderlich.com"
      ]
      let format = UIGraphicsPDFRendererFormat()
      format.documentInfo = pdfMetaData as [String: Any]

      // 2
      let pageWidth = 8.5 * 72.0
      let pageHeight = 11 * 72.0
      let pageRect = CGRect(x: 0, y: 0, width: pageWidth, height: pageHeight)

      // 3
      let renderer = UIGraphicsPDFRenderer(bounds: pageRect, format: format)
      // 4
      let data = renderer.pdfData { (context) in
        // 5
        context.beginPage()
        // 6
        let attributes = [
          NSAttributedString.Key.font: UIFont.boldSystemFont(ofSize: 72)
        ]
        let text = "I'm a PDF!"
        text.draw(at: CGPoint(x: 0, y: 0), withAttributes: attributes)
      }

      return data
    }
    
    func saveDataToFile(data: Data, filePath: String) {
        do {
            // Create a URL for the file path
            let fileURL = URL(fileURLWithPath: filePath)
            
            // Write the data to the file
            try data.write(to: fileURL)
            
            print("Data saved successfully at: \(fileURL.path)")
        } catch {
            print("Failed to save data: \(error.localizedDescription)")
        }
    }
    
    func documentsDirectoryPath() -> String? {
        let paths = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)
        return paths.first
    }

}