Watermarking a PDF on iOS

let url = ... // Replace this with a URL pointing to the PDF you want to watermark.
guard let document = PDFDocument(url: url) else {
    NSLog("Unable to create PDF document")
    return
}

// `page` is of the type `PDFPage`.
let page = document.page(at: 0)!
// Extract the crop box of the PDF. We need this to create an appropriate graphics context.
let bounds = page.bounds(for: .cropBox)

// Set up a `UIGraphicsImageRenderer` to handle creation of a watermarked image.
let renderer = UIGraphicsImageRenderer(bounds: bounds, format: UIGraphicsImageRendererFormat.default())

// Call the `image(actions:)` method of `UIGraphicsImageRenderer`. This method takes a block in which
// you can do any drawing you want and get a `UIImage` with the result.
let image = renderer.image { (context) in
    // We transform the CTM to match the PDF's coordinate system, but only long enough to draw the page.
    context.cgContext.saveGState()

    context.cgContext.translateBy(x: 0, y: bounds.height)
    context.cgContext.concatenate(CGAffineTransform.init(scaleX: 1, y: -1))
    page.draw(with: .cropBox, to: context.cgContext)

    context.cgContext.restoreGState()

    context.cgContext.translateBy(x: 50, y: bounds.size.height / 2)
    let attributes: [NSAttributedString.Key: Any] = [
        .font: UIFont.boldSystemFont(ofSize: 50),
        .foregroundColor: UIColor.red.withAlphaComponent(0.5)
    ]

    let text = "Watermark on the PDF Page!"
    // Draw the text with the attributes from above.
    text.draw(with: bounds, options: .usesLineFragmentOrigin, attributes: attributes, context: NSStringDrawingContext())
}

// Create a new `PDFPage` with the image that was generated above.
let newPage = PDFPage(image: image)!

// The annotations need to be transferred from the old page to the new one.
for annotation in page.annotations {
    newPage.addAnnotation(annotation)
}

// Insert the new page at index 0.
document.insert(newPage, at: 0)
// Remove the page at index 1. This is the page that was at index 0 but got moved.
document.removePage(at: 1)

let document: PDFDocument = ... // This is the document we added the watermark to.
document.write(toFile: <#a_path_on_disk#>)

let document: PSPDFDocument = ... // An instance of PSPDFDocument.

let renderDrawBlock: RenderDrawHandler = { context, page, cropBox, _, _ in
    // Since the page index is passed in, we can draw this either on specific pages or on all of them.

    let text = "PSPDF Live Watermark"

    // Add text over the diagonal of the page.
    context.translateBy(x: 50, y: cropBox.size.height / 2)
    let attributes: [NSAttributedString.Key: Any] = [
        .font: UIFont.boldSystemFont(ofSize: 100),
        .foregroundColor: UIColor.red.withAlphaComponent(0.5)
    ]
    text.draw(with: cropBox, options: .usesLineFragmentOrigin, attributes: attributes, context: NSStringDrawingContext())
}

document.updateRenderOptions([.draw(renderDrawBlock)], type: .all)

// Create default configuration.
guard let configuration = PSPDFProcessorConfiguration(document: document) else {
    // Handle failure.
    abort()
}

configuration.drawOnAllCurrentPages { context, page, cropBox, _, _ in
    let text = "PSPDF Live Watermark"

    // Add text over the diagonal of the page.
    context.translateBy(x: 50, y: cropBox.size.height / 2)
    let attributes: [NSAttributedString.Key: Any] = [
        .font: UIFont.boldSystemFont(ofSize: 100),
        .foregroundColor: UIColor.red.withAlphaComponent(0.5)
    ]
    text.draw(with: cropBox, options: .usesLineFragmentOrigin, attributes: attributes, context: NSStringDrawingContext())
}

do {
    // Start the conversion from `document` to `processedDocumentURL`.
    let processor = PSPDFProcessor(configuration: configuration, securityOptions: nil)
    try processor.write(toFileURL: processedDocumentURL)
} catch {
    // Handle failure.
    abort()
}