Dispatch several download tasks

I'm building an iOS app that will download 300 pdf files each one with more or less 3mb in size and I have limited understanding of Grand Central Dispatch.

I came across a really nice tutorial with code that I'm using:
Tutorial:Guide link
Code:Code example


The problem is  that the app will freeze for some time, when running this code to add all the 300 tasks:

-(void)startAllDownloads{

    for (int i=0; i<[self.arrFileDownloadData count]; i++) {
        FileDownloadInfo *fdi = [self.arrFileDownloadData objectAtIndex:i];


        // Check if a file is already being downloaded or not.

        if (!fdi.isDownloading) {

            // Check if should create a new download task using a URL, or using resume data.
            if (fdi.taskIdentifier == -1) {
                //new NSURLSessionDownloadTask
                fdi.downloadTask = [self.session downloadTaskWithURL:[NSURL URLWithString:fdi.downloadSource]];
            }
            else{

                fdi.downloadTask = [self.session downloadTaskWithResumeData:fdi.taskResumeData];

            }

            // Keep the new taskIdentifier.
            fdi.taskIdentifier = fdi.downloadTask.taskIdentifier;

            NSLog(@"starting download id:%lu",fdi.taskIdentifier);

            // Start the download.
            //dispatch_async(dispatch_get_main_queue(), ^{
            [fdi.downloadTask resume];
            //});

            // Indicate for each file that is being downloaded.
            fdi.isDownloading = YES;
        }
    }
}

I have tried with a dispatch_async but with no success,  It always freezes.

Sorry if I'm not being very clear, If more info is needed just ask :)