AndroidExternalStorageWriter.cs

    public class AndroidExternalStorageWriter : IAndroidExternalStorageWriter
    {
        public string CreateFile(string filename, byte[] bytes)
        {
            if (!Directory.Exists(Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, "AccountMate")))
                Directory.CreateDirectory(Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, "AccountMate"));

            var path = Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, "AccountMate", filename);

            File.WriteAllBytes(path, bytes);

            return path;
        }
    }

DocumentViewer.cs

    public class DocumentViewer : IDocumentViewer
    {
        public void ShowDocumentFile(string filepath, string mimeType)
        {
            var uri = Android.Net.Uri.Parse("file://" + filepath);
            var intent = new Intent(Intent.ActionView);
            intent.SetDataAndType(uri, mimeType);
            intent.SetFlags(ActivityFlags.ClearWhenTaskReset | ActivityFlags.NewTask);
            Forms.Context.StartActivity(Intent.CreateChooser(intent, "Select App"));
        }

        public DocumentViewer()
        {
            
        }
    }

Globals.cs

public static async void ViewFile(byte[] fileBytes, string filename)
        {
            var filepath = "";

            if (Device.OS == TargetPlatform.Android)
            {
                var androidWriter = Resolver.Resolve<IAndroidExternalStorageWriter>();
                filepath = androidWriter.CreateFile(filename, fileBytes.ToArray());
            }
            else if (Device.OS == TargetPlatform.iOS)
            {
                using (var pdfBytes = new MemoryStream(fileBytes))
                {
                    var rootFolder = FileSystem.Current.LocalStorage;
                    var file = await rootFolder.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
                    filepath = file.Path;
                    var newFile = await file.OpenAsync(FileAccess.ReadAndWrite);

                    using (var outputStream = newFile)
                        pdfBytes.CopyTo(outputStream);
                }
            }

            if (string.IsNullOrEmpty(filepath))
                return;

            var documentViewer = Resolver.Resolve<IDocumentViewer>();

            var mimeType = "application/vnd.openxmlformats-officedocument.wordprocessingml.document";

            if (Path.GetExtension(filename).ToLower() == ".pdf")
                mimeType = "application/pdf";

            try
            {
                documentViewer.ShowDocumentFile(filepath, mimeType);
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex.Message);
            }
        }

IandroidExternalStorageWriter.cs

    public interface IAndroidExternalStorageWriter
    {
        string CreateFile(string filename, byte[] bytes);
    }

IDocumentViewer

    public interface IDocumentViewer
    {
        void ShowDocumentFile(string filepath, string mimeType);
    }

iOSDocumentViewer.cs

    public class DocumentViewer : IDocumentViewer
    {
        public DocumentViewer()
        {
            
        }

        public void ShowDocumentFile(string filepath, string mimeType)
        {
            var fileinfo = new FileInfo(filepath);
            var previewController = new QLPreviewController
            {
                DataSource = new PreviewControllerDataSource(fileinfo.FullName, fileinfo.Name)
            };

            var controller = FindNavigationController();

            controller?.PresentViewController(previewController, true, null);
        }

        private UINavigationController FindNavigationController()
        {
            foreach (var window in UIApplication.SharedApplication.Windows)
            {
                if (window.RootViewController.NavigationController != null)
                {
                    return window.RootViewController.NavigationController;
                }

                var value = CheckSubs(window.RootViewController.ChildViewControllers);
                if (value != null)
                    return value;
            }

            return null;
        }

        private UINavigationController CheckSubs(UIViewController[] controllers)
        {
            foreach (var controller in controllers)
            {
                if (controller.NavigationController != null)
                {
                    return controller.NavigationController;
                }

                var value = CheckSubs(controller.ChildViewControllers);

                return value;
            }

            return null;
        }
    }

    public class DocumentItem : QLPreviewItem
    {
        private readonly string _uri;

        public DocumentItem(string title, string uri)
        {
            ItemTitle = title;
            _uri = uri;
        }

        public override string ItemTitle { get; }

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

    public class PreviewControllerDataSource : QLPreviewControllerDataSource
    {
        private readonly string _url;
        private readonly string _filename;

        public PreviewControllerDataSource(string url, string filename)
        {
            _url = url;
            _filename = filename;
        }

        public override IQLPreviewItem GetPreviewItem(QLPreviewController controller, nint index)
        {
            return new DocumentItem(_filename, _url);
        }

        public override nint PreviewItemCount(QLPreviewController controller)
        {
            return 1;
        }
    }

Thanks man, well done!