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

dropboxpath is /file_name_with_extension

yeah i am sure 

from retrieving from this

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

        NSLog(@"\n\naccess token key: %@ secret: %@", creds.accessToken, creds.accessTokenSecret);

and when i pass creds.acessToken as <acessToken>

{"error" : "Invalid OAuth2 token."}

and when i pass creds.accessTokenSecret as <acessToken>

{"error" : "Invalid OAuth2 token."}

can you please elaborate the whole method for getting access token for https://content.dropboxapi.com/1/previews/auto as you can see i have already tried the above method.

my scenario is when a user login to app then the access token should get saved in the app

Unfortunately, I don't have a sample implementation of this endpoint in Objective C to share. 

Note that the iOS SDK itself uses OAuth 1, so the stored access token from credentialStoreForUserId can't be directly used as an OAuth 2 access token, which is why you're getting that "Invalid OAuth2 token" error.

If you do want to use that stored OAuth 1 access token, there's a blog post that shows how to implement OAuth 1 here:

https://blogs.dropbox.com/developers/2012/07/using-oauth-1-0-with-the-plaintext-signature-method/

(The iOS Core SDK is open source though, so you could alternatively add the source directly and modify the code to add /previews.)

Another option entirely though, which I recommend if your app is still in development mode, is to switch to using the SwiftyDropbox library for API v2, if you can (since it's for Swift):

https://www.dropbox.com/developers/documentation/swift

I tried the whole procedure
as documented in 

https://blogs.dropbox.com/developers/2012/07/using-oauth-1-0-with-the-plaintext-signature-method/

but when i passed the end resulted access token to 

https://www.dropbox.com/developers-v1/core/docs#previews

the error is still same

i told you i am using objective-c  for development and all the issue 
and dear you just wasted my day

That error message indicates you're still passing up the access token like an OAuth 2 access token (using the "Bearer" syntax), but if you have an OAuth 1 access token, you need to use the OAuth 1 syntax from the blog post I linked to.

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.)

after how much time the Access Token Generated by this blog post  will expire

Dropbox API access tokens don't expire. 

Users can manually revoke tokens themselves though, e.g. via:

https://www.dropbox.com/account/security

Apps can also revoke access tokens via:

v1: https://www.dropbox.com/developers/core/docs#disable-token

v2: https://www.dropbox.com/developers/documentation/http/documentation#auth-token-revoke