PdfFile.swift

struct PdfFile {
    var filepath: String?
    var file: PDFDocument?
    private var cacheImages = NSCache<NSString, UIImage>()

    var numberOfPages: Int {
        return file?.pdfDocument?.numberOfPages ?? 0
    }

    init(filepath path: String?) {
        filepath = path
        file = PDFDocument.open(file: path)
    }

    mutating func page(forIndex index: Int, scaleFactor: CGFloat) -> UIImage? {
        guard let pdf = file else { return nil }
        let stringIndex = String(index) as NSString
        if let image = cacheImages.object(forKey: stringIndex) {
            return image
        } else {
            let page = index + 1
            let image = pdf.render(page: page, scaleFactor: scaleFactor)
            if let image = image {
                cacheImages.setObject(image, forKey: stringIndex)
            }
            return image
        }
    }
}


pages = PdfFile(filepath: "/some/path")

DispatchQueue.global(qos: .background).async { in
       let image = pages.page(forIndex: index, scaleFactor: strongSelf.scaleFactor)
        DispatchQueue.main.async { () -> Void in
             completion(image)
         }
  }