when we want to open the pdf file from a URL string then we use the UIWebView. In webview we can directly open the file by loading the url request.use the following function--

-(void)addWebViewOverLay:(UIWebView *)webView {
    webView = [[UIWebView alloc] initWithFrame:CGRectMake(x, y, width  , height)];

    UIActivityIndicatorView *actInd=[[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];

    //Change the color of the indicator, this override the color set by UIActivityIndicatorViewStyleWhiteLarge
    actInd.color=[UIColor blackColor];

    //Put the indicator on the center of the webview
    [actInd setCenter:webView.center];

    //Assign it to the property
    activityIndicator=actInd;
    [activityIndicator setHidesWhenStopped:YES];
    //Add the indicator to the webView to make it visible
    [webView addSubview:activityIndicator];

    webView.delegate=self;
    [webView setBackgroundColor:[UIColor grayColor]];
    NSURL *targetURL = [NSURL URLWithString:@"Enter URL Here"];
    NSURLRequest *request = [NSURLRequest requestWithURL:targetURL];
    [webView loadRequest:request];
    [activityIndicator startAnimating];

    [webView setScalesPageToFit:YES];
    [[webView layer] setCornerRadius:10];
    [webView setClipsToBounds:YES];
    // Create colored border using CALayer property
    [[webView layer] setBorderColor:
     [[UIColor colorWithRed: 43/255.0 green:124/255.0 blue:24/255.0 alpha:1.0] CGColor]];

    [[webView layer] setBorderWidth:2.75];
    UIButton *closeButton = [UIButton buttonWithType:UIButtonTypeCustom];
    closeButton.tag=2222;
    [closeButton setFrame:CGRectMake(webView.frame.size.width-5, 5, 26, 26)];
    [closeButton setImage:[UIImage imageNamed:@"icon_close.png"] forState:UIControlStateNormal];
    [closeButton addTarget:self action:@selector(closeWebView:) forControlEvents:UIControlEventTouchUpInside];

    [self.view addSubview:webView];
    [[self view] addSubview:closeButton];
}

here we use delegate of uiwebview to stop the activity indicator.

-(void)webViewDidFinishLoad:(UIWebView *)webView {
    [activityIndicator stopAnimating];
}