Enable zooming/pinch on UIWebView

Make sure you checked "Scales page to fit"

you can use webView.scalesPageToFit=YES; programmatically

If you are using in xib than just click the check box "Scaling" scales Page to fit

This Logic for zooming of UIWebView, no need to add UIWebView on UIScrollView

Well only problem with webView.scalesPageToFit = YES; is that, it will change initial content of font size but I found other option

Add <UIWebViewDelegate, UIScrollViewDelegate> to your .h file

Creation of your UIWebView.

self.mWebview = [[UIWebView alloc] init];
self.mWebview.delegate = self; /// set delegate method of UIWebView
self.mWebview.frame = CGRectMake(0, 35, self.view.bounds.size.width, self.view.bounds.size.height - 80); // set frame whatever you want..
[self.mWebview setOpaque:NO];
self.mWebview.backgroundColor = [UIColor clearColor];
[self.view addSubview:self.mWebview];

With load HTML file/content.

NSString* htmlString = [NSString stringWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"File Name"ofType:@"html"] encoding:NSUTF8StringEncoding error:nil];
[self.mWebview loadHTMLString:htmlString baseURL:[NSURL fileURLWithPath:[[NSBundle mainBundle] bundlePath]]];


#pragma mark -
#pragma mark - Webview Delegate Methods

- (void) webViewDidFinishLoad:(UIWebView *)webView
{
    webView.scrollView.delegate = self; // set delegate method of UISrollView
    webView.scrollView.maximumZoomScale = 20; // set as you want.
    webView.scrollView.minimumZoomScale = 1; // set as you want.

    //// Below two line is for iOS 6, If your app only supported iOS 7 then no need to write this.
    webView.scrollView.zoomScale = 2;
    webView.scrollView.zoomScale = 1;
}

#pragma mark -
#pragma mark - UIScrollView Delegate Methods

- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale
{
    self.mWebview.scrollView.maximumZoomScale = 20; // set similar to previous.
}

NOTE: I had to tested on Mac OS X - 10.9.3 with Xcode 5.1.1 and iOS version 6.1 and latter.

I hope this will helpful for you. :)