Thanks for your question. As I understand it, you have Word documents (.doc) and Excel workbooks (.xls, .xlsx) in your Media Library and want to replace them with (or add) a PDF version; is that right?
If so, I regret that MLA does not provide any features for converting Media Library items from one format to another or for replacing the file associated with a Media Library item. Changing the file type (and MIME type) of existing items would be problematic since MIME type is stored in the database and used in a variety of ways. Converting a file from one format to another requires tools not provided by WordPress.
I wish the news were better, but I am marking this topic resolved because your question has been answered to the best of my ability. I hope you can find a suitable solution for your application. Thanks for your interest in the plugin.
