unfortunately this is not possible at the moment. However, if the documents will appear on a page inside of a download list, you can set the keywords in the description field and use PageIndex. That way when you search for a keyword, the page which contains the DownloadList with the documents that match the criteria will appear as one of the results.
We are planning on implementing a search index that will search the actual content of PDF and WORD documents, so once that is implemented you will not need to tag your documents, because the search will be performed on the document itself.
I hope I was able to help you for the moment. Thank you for getting in touch with us.
the Telerik team