The Random document web part selects random documents from the specified location and displays them based on the assigned ASCX transformation.

You need to define the set of documents from which you want to choose the random document by setting the Path property to e.g. /Products/% and setting the Document type property e.g. to cms.product. The document will be displayed using the transformation specified in the Transformation name value - e.g. cms.product.preview.

Help URL: http://devnet.kentico.com/docs/webparts/index.html?randomDocument_overview.htm