FilePreview is a easy way to open in-app some kind of files (PDF, DOC, XLS, TXT, images, etc) as read only. This is a simple implementation using WKWebView but with an interface to check if is possible to open the file before showing the ViewController.

FilePreview.swift

//
//  FilePreview.swift
//  COYO Messenger Framework
//
//  Created by Bruno Guidolim on 21.11.18.
//  Copyright © 2018 COYO. All rights reserved.
//

import Foundation
import WebKit

final public class FilePreview: NSObject {

    public typealias CompletionHandler = ((_ canShowFile: Bool, _ viewController: FilePreviewController?) -> Void)

    private let webView: WKWebView
    private let fileURL: URL
    private var completionHandler: CompletionHandler?

    init(fileURL: URL) {
        self.fileURL = fileURL
        self.webView = WKWebView(frame: .zero)
        super.init()
        webView.navigationDelegate = self
    }

    public func canShowFile(completion: CompletionHandler?) {
        self.completionHandler = completion
        self.webView.loadFileURL(self.fileURL, allowingReadAccessTo: self.fileURL.deletingLastPathComponent())
    }
}

extension FilePreview: WKNavigationDelegate {

    public func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void) {
        if let url = navigationAction.request.url, url != fileURL {
            UIApplication.shared.open(url)
            decisionHandler(.cancel)
            return
        }
        decisionHandler(.allow)
    }

    public func webView(_ webView: WKWebView, decidePolicyFor navigationResponse: WKNavigationResponse, decisionHandler: @escaping (WKNavigationResponsePolicy) -> Void) {
        if navigationResponse.canShowMIMEType {
            let vc = FilePreviewController(webView: webView, title: fileURL.lastPathComponent)
            completionHandler?(true, vc)
            decisionHandler(.allow)
            return
        }
        completionHandler?(false, nil)
        decisionHandler(.cancel)
    }
}

public protocol FilePreviewControllerDelegate: class {
    func filePreviewControllerDidTapShareButton(_ viewController: UIViewController)
}

final public class FilePreviewController: UIViewController {

    public weak var delegate: FilePreviewControllerDelegate?

    private let webView: WKWebView

    fileprivate init(webView: WKWebView, title: String) {
        self.webView = webView
        super.init(nibName: nil, bundle: nil)
        self.title = title
    }

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

    override public func loadView() {
        view = webView
    }

    override public func viewDidLoad() {
        super.viewDidLoad()
        if #available(iOS 11, *) {
            webView.scrollView.contentInsetAdjustmentBehavior = .always
        } else {
            self.automaticallyAdjustsScrollViewInsets = true
        }

        let shareButton = UIBarButtonItem(barButtonSystemItem: .action, target: self, action: #selector(shareButtonTapped(_:)))
        navigationItem.setRightBarButton(shareButton, animated: true)
    }

    @objc private func shareButtonTapped(_ sender: Any) {
        delegate?.filePreviewControllerDidTapShareButton(self)
    }
}

How do you call this from another file to show the view?

How do you call this from another file to show the view?

let filePreview = FilePreview(fileURL: fileURL)
filePreview.canShowFile { [weak self] canShowFile, fileViewController in
    if canShowFile {
        self?.present(fileViewController, animated: true)
   } else {
        // Share activity code for example
   }
}

this is not loading file saved in document directory.