NSData *myData = [NSData dataWithContentsOfFile:_filename];
NSArray *activityItems = [NSArray arrayWithObjects:myData, nil];
UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:nil applicationActivities:nil];
activityViewController.excludedActivityTypes 
 
= @[UIActivityTypeCopyToPasteboard];
[self presentViewController:activityViewController animated:YES completion:nil];

On iPad the activity view controller will be displayed as a popover using the new UIPopoverPresentationController, it requires that you specify an anchor point for the presentation of the popover using one of the three following properties:

In order to specify the anchor point you will need to obtain a reference to the UIActivityController’s UIPopoverPresentationController and set one of the properties as follows:

if ( [activityViewController respondsToSelector:@selector(popoverPresentationController)] ) { 
// iOS8
 activityViewController
 
.popoverPresentationController.sourceView =
parentView;
 }

You aren’t initialising the activityViewController on iPad so it will always be nil.

Try:

- (void)shareLeaflet
{
    NSString *forwardedString = [[NSString alloc] initWithFormat:@"Check out this leaflet\n\n %@ \n\nself.theURLToShare];
    UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:[NSArray arrayWithObjects:forwardedString, nil] applicationActivities:nil];

    if (IDIOM == IPAD)
    {
        NSLog(@"iPad");
        activityViewController.popoverPresentationController.sourceView = self.view;
//        activityViewController.popoverPresentationController.sourceRect = self.frame;
        [self presentViewController:activityViewController
                           animated:YES
                         completion:nil];
    }
    else
    {
        NSLog(@"iPhone");
        [self presentViewController:activityViewController 
                          animated:YES 
                        completion:nil];
    }

And then to display it like in the image: (_shareBarButton is the UIBarButtonItem that you want the popover to display from)

- (void)shareLeaflet
    {
        NSString *forwardedString = [[NSString alloc] initWithFormat:@"Check out this leaflet\n\n %@ \n\nself.theURLToShare];
        UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:[NSArray arrayWithObjects:forwardedString, nil] applicationActivities:nil];

        if (IDIOM == IPAD)
        {
            NSLog(@"iPad");
            activityViewController.popoverPresentationController.sourceView = self.view;
    //        activityViewController.popoverPresentationController.sourceRect = self.frame;

           _popover = [[UIPopoverController alloc] initWithContentViewController:activityViewController];
           _popover.delegate = self;
           [_popover presentPopoverFromBarButtonItem:_shareBarButton permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
        }
        else
        {
            NSLog(@"iPhone");
            [self presentViewController:activityViewController 
                              animated:YES 
                            completion:nil];
        }

The way I use it

#define IDIOM    UI_USER_INTERFACE_IDIOM()
#define IPAD     UIUserInterfaceIdiomPad

if ( IDIOM == IPAD ) {
    /* do something specifically for iPad. */
} else {
    /* do something specifically for iPhone or iPod touch. */
}   

Other Examples

if ( [(NSString*)[UIDevice currentDevice].model hasPrefix:@"iPad"] ) {
    return YES; /* Device is iPad */
}

#define IPAD     UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad
if ( IPAD ) 
     return YES;