A UIView that renders a PDF page at a given resolution and constrains itself to the PDF page's aspect ratio

PDFPageView.swift

// Uses Cartography for layout, but you should be able to translate it to NSLayoutConstraint pretty easily
import Cartography

/// Render a single page of a PDF, non-interactive, at an arbitrary size.
/// Adds its own constraint to maintain its correct aspect ratio.
class PDFPageView: UIView {
  var displayBox: PDFDisplayBox = .artBox
  var majorAxisLength: CGFloat = 2048
  var page: PDFPage? { didSet { applyState() } }

  private let aspectRatioConstrainGroup = ConstraintGroup()

  convenience init(page: PDFPage?) {
    self.init(frame: .zero)
    self.page = page
  }

  override init(frame: CGRect) {
    super.init(frame: frame)
  }

  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

  private func applyState() {
    if let page = page {
      let bounds = page.bounds(for: displayBox)
      let multiplier = bounds.size.width / bounds.size.height
      constrain(self, replace: aspectRatioConstrainGroup) { it in
        it.width == it.height * multiplier
      }
      setNeedsDisplay()
      setNeedsLayout()
    } else {
      // no constraints at all (no PDF, so no 'correct' aspect ratio)
      constrain(self, replace: aspectRatioConstrainGroup) { _ in }
    }
  }

  // MARK: UIView

  override var intrinsicContentSize: CGSize {
    if let page = page {
      let bounds = page.bounds(for: displayBox)
      if bounds.size.width > bounds.size.height {
        return CGSize(
          width: majorAxisLength,
          height: majorAxisLength * (bounds.size.height / bounds.size.width))
      } else {
        return CGSize(
          width: majorAxisLength * (bounds.size.width / bounds.size.height),
          height: majorAxisLength)
      }
    } else {
      return .zero
    }
  }

  override func draw(_ rect: CGRect) {
    guard let context = UIGraphicsGetCurrentContext(), let page = page else { return }
    // Render the PDF to the CGContext.

    // PDF coordinate system is Y-flipped from Core Graphics
    context.translateBy(x: 0, y: rect.height)

    // Ask PDF to apply its crop box transform
    let bounds = page.bounds(for: displayBox)
    page.transform(context, for: displayBox)

    // Scale PDF to view size
    context.scaleBy(x: rect.width / bounds.width, y: -rect.height / bounds.height)

    // Draw!
    page.draw(with: displayBox, to: context)
  }
}