I have integrated my salesforce org with BOX. And tried to upload the file from salesforce to box using upload file api in apex. And i successfully uploaded it in desired folder but then i am not able to preview those files in box. Why is it so? It is showing a error message , for pdf :- 

This preview didn’t load because the document may be protected. Please try to open or download the file to view.

For doc:- This preview didn’t load. Please try to open or download the file to view. 

I have tried uploading many files of same type but i am not able to view them.