Google recently introduced the new X-Robots-Tag directive to allow webmasters to control access to non-webpage documents, such as PDF files, audio and video files, that doesn’t mean it will help to increase search engine ranking for your website or blog. The X-Robots-Tag is included in the HTTP header of a document. The new X-Robots-Tag allows webmasters to add information about search engine indexing to the HTTP header.
Last week only, Yahoo announced that they now support the X-Robots-Tag in the HTTP header of a document. This new tag allows you to influence how and which WebPages Google and Yahoo indexes. The X-Robots-Tag is currently supported by Google and Yahoo only; other search engines don't support the tag yet.
X-Robots-Tag supports the following commands:
For further details you can the read at the link below