The configuration variable MS_THUMBNAIL can be used to generate a thumbail of the last view of the DGN. Unless the Saved View 'preview' exists in then the DGN, then the thumbnail will be the 'preview' thumbnail.

The idea is to be able to use the DGN thumbnail, or to render a new thumbnail using the PDF.

Using the pdf.pltcfg driver configuration. Currently when I configure the pdf driver there is no option for a thumbnail. ProjectWise used to render a thumbnail.