AndroidDocumentViewer.cs

    public class DocumentViewer : IDocumentViewer
    {
        public void ShowDocumentFile(string filepath, string mimeType)
        {
            var uri = global::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);
        }

        public DocumentViewer()
        {
            
        }
    }

HowToUse.cs

                var documentViewer = DependencyService.Get<IDocumentViewer>();

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

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

                documentViewer.ShowDocumentFile(filepath, mimeType);

IDocumentViewer.cs

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

iosDocumentViewer

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 window = UIApplication.SharedApplication.KeyWindow;
            var vc = window.RootViewController;
            while (vc.PresentedViewController != null)
            {
                vc = vc.PresentedViewController;
            }

            vc?.PresentViewController(previewController, true, 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;
        }
    }