PdfService.cs

using Foundation;

using QuickLook;

using System;
using System.Threading.Tasks;

using Xamarin.Essentials;

namespace Test.iOS
{
	public sealed class PdfService
	{
		// filePath = Xamarin.Essentials.FileSystem.CacheDirectory + FileName (file.pdf)
		
		public Task OpenAsync(string fileName, string filePath)
		{
			var currentController = Platform.GetCurrentUIViewController();
			var qLPreviewController = new QLPreviewController();
			var item = new PreviewItem(fileName, filePath);

			qLPreviewController.DataSource = new PreviewControllerDataSource(item);
			currentController.PresentViewController(qLPreviewController, true, null);
			
			return Task.CompletedTask;
		}

		private sealed class PreviewItem : QLPreviewItem
		{
			private string _fileName;
			private string _filePath;

			public PreviewItem(string fileName, string filePath)
			{
				_fileName = fileName;
				_filePath = filePath;
			}

			public override string ItemTitle => _fileName;

			public override NSUrl ItemUrl => NSUrl.FromFilename(_filePath);
		}

		private sealed class PreviewControllerDataSource : QLPreviewControllerDataSource
		{
			private QLPreviewItem _item;

			public PreviewControllerDataSource(QLPreviewItem item) => _item = item;

			public override nint PreviewItemCount(QLPreviewController controller) => 1;

			public override IQLPreviewItem GetPreviewItem(QLPreviewController controller, nint index) => _item;
		}
	}
}