The custom field's value is lost when MS Document file is uploaded at second time to update in Document Library.

Hi,

I have a Document Library and I added Author field and 'About Author' field in the library.
I uploaded a ms document and the author files is automatically fillled with 'Authors metadta' from 
document.  I updated the values of Title and 'About Author' field in Edit Form.
I try to uploaded the same document again. At that time, Title's value is lost because the orginal file does not have Title's value. But I think my 'About Author' value that i updated should not be lost.
I try with pdf file that does not have metadata files as ms document. I updated the information of fields. I uploaded the same pdf file second time. This custom field's values are not lost.


Any one please give me suggestion not to lost custom field's value at ms document at updating.

Thanks!