If the format conversion for the document file uploaded by you fails, then you should do one of the below things:
1) Edit the document to upload a new file.
2) Delete the document entry and create a completely new document entry.

The possible reasons why format conversion for your document might not be getting completed could be that the document might be password protected or copyrighted. The formats for password protected or copyrighted documents cannot be converted. Hence, please ensure that your document is not password protected or copyrighted by someone else.