how to get all files in pdf format can dropbox convert before i download it through the API

For example, I'm not an Objective-C expert, but something like this (modified from your previous code sample):

    MPOAuthCredentialConcreteStore *creds = [[DBSession sharedSession] credentialStoreForUserId:UID];




    NSString *authorization = [NSString stringWithFormat:@"OAuth oauth_version=\"1.0\", oauth_signature_method=\"PLAINTEXT\", oauth_consumer_key=\"%@\", oauth_token=\"%@\", oauth_signature=\"%@&%@\", appKey, creds.accessToken, appSecret, creds.accessTokenSecret];




    NSString *filePath = @"/Testing/Documents/test.docx";

    NSURL *url = [NSURL URLWithString:[

                                       [NSString stringWithFormat:@"https://content.dropboxapi.com/1/previews/auto/%@", filePath] stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding

                                       ]

                  ];







    NSMutableURLRequest *request =

    [NSMutableURLRequest requestWithURL:url

                        cachePolicy:NSURLRequestReloadIgnoringLocalAndRemoteCacheData

                        timeoutInterval:10

     ];




    [request setValue:authorization forHTTPHeaderField:@"Authorization"];

    [request setHTTPMethod: @"GET"];




    NSError *requestError = nil;

    NSURLResponse *urlResponse = nil;




    NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:&urlResponse error:&requestError];




    NSString *fileName = @"test_205881743.pdf";

    NSString *localDir = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];

    NSString *localFilePath = [localDir stringByAppendingPathComponent:fileName];




    [response writeToFile:localFilePath atomically:YES];

    NSLog(@"%@", localFilePath);

(That doesn't have the error handling implemented though.)