UIWebView category method for loading HTML from the resource bundle. Content will be loaded such that the resource bundle directory is the base URL, so relative links (spreadsheets, images, etc.) work properly.

UIWebView+XXXLoadHTMLResourceNamed.h

#import <UIKit/UIKit.h>

@interface UIWebView (XXXLoadHTMLResourceNamed)

// Load content from HTML resource file with specified name.
//
// Resource will be loaded using URL, and base URL will be the
// resource bundle directory, so relative paths will work properly.

- (void)loadHTMLResourceNamed:(NSString *)resourceName;

@end

UIWebView+XXXLoadHTMLResourceNamed.m

#import "UIWebView+XXXLoadHTMLResourceNamed.h"

@implementation UIWebView (XXXLoadHTMLResourceNamed)

// Load content from HTML resource file with specified name.
//
// Resource will be loaded using URL, and base URL will be the
// resource bundle directory, so relative paths will work properly.

- (void)loadHTMLResourceNamed:(NSString *)resourceName {
    NSBundle *bundle = [NSBundle mainBundle];
    NSURL *htmlFileURL = [bundle URLForResource:resourceName
                                  withExtension:@"html"];
    
    NSError *error;
    NSString *htmlContent = [NSString stringWithContentsOfURL:htmlFileURL
                                                     encoding:NSUTF8StringEncoding
                                                        error:&error];
    if (error) {
        NSLog(@"%s: Unable to load HTML: %@",
              __PRETTY_FUNCTION__, [error localizedDescription]);
    }
    else {
        NSURL *baseURL = [NSURL fileURLWithPath:[bundle resourcePath]
                                    isDirectory:YES];
        [self loadHTMLString:htmlContent baseURL:baseURL];
    }
}

@end