Cannot find 'self' in scope

Hi! I'm trying to display a pdf inside my iOS app and I'm using the code displayed below, but for every self inside it I'm getting this error: Cannot find 'self' in scope

//
//  gimnaziumStundenplan.swift
//  stundenplan
//
//  Created by Alex on 04/09/2021.
//

import UIKit
import PDFKit

private func createPdfView(withFrame frame: CGRect) -> PDFView {
    let pdfView = PDFView(frame: frame)
    pdfView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
    pdfView.autoScales = true

    return pdfView
}

private func createPdfDocument(forFileName fileName: String) -> PDFDocument? {
    if let resourceUrl = self.resourceUrl(forFileName: fileName) {
        return PDFDocument(url: resourceUrl)
    }
    

    if let resourceUrl = URL(string: "https://web.stanford.edu/class/archive/cs/cs161/cs161.1168/lecture4.pdf") {
            return PDFDocument(url: resourceUrl)
        }

        return nil

}



private func displayPdf() {
    let pdfView = self.createPdfView(withFrame: self.view.bounds)
    if let pdfDocument = self.createPdfDocument(forFileName: "heaps") {
        self.view.addSubview(pdfView)
        pdfView.document = pdfDocument
    }
}
}