iOS PDF文件连接转化为img图片保存到相册

{

    //self.passURL 是后台返回的可以加载显示到webView中的PDF链接
    if ([self.passURL isEqualToString:@""] || self.passURL == nil || [self.passURL isEqual:nil] || !self.passURL) {
        [self InvalidAlertView];
        return;
    }
    NSURL *url = [[NSURL alloc] initWithString:self.passURL];
    CFURLRef ref = (__bridge CFURLRef)url;
    pdf = CGPDFDocumentCreateWithURL(ref);
    CFRelease(ref);
    
    CGImageRef imageRef = PDFPageToCGImage(1,pdf);
    UIImage *amg = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    
    //保存图片到相册
    UIImageWriteToSavedPhotosAlbum(amg, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
    
}

- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo{
    
    if (error == nil) {
        
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"提示" message:@"图片已保存到相册" delegate:self cancelButtonTitle:nil otherButtonTitles:@"好的", nil];
        [alert show];
    }else{
        
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"提示" message:@"保存失败" delegate:self cancelButtonTitle:nil otherButtonTitles:@"确定", nil];
        [alert show];
    }
    
}

问题：在dealloc中 写CGPDFDocumentRelease(pdf)这个方法，如果保存完照片后，快速点击返回按钮会崩溃，如果等2秒后点击返回不会崩，所以在项目中先屏蔽掉了。

如果有哪位解决了不崩溃，请留个言，谢过了

- (void)dealloc {

   // CGPDFDocumentRelease(pdf);

    if ([self.mainWebView isLoading]){

        [self.mainWebView stopLoading];

    }

}