When you create a PDF file with the standard pdf.pltcfg settings from a drawing, which includes a raster file. The raster file will be part of the Images Layer in the pdf file.
How to display a Raster file below the level layer in the PDF file
Set in the pdf.pltcfg printer driver configuration file under the Base Properties TAB -> Advanced -> Enable merging of background Raster = false
MicroStation -> File -> Print…-> the Print Dialog box opens -> File -> Select Bentley Driver -> select the Bentley driver pdf.pltcfg -> open -> the print dialog box is open
Print dialog box -> file -> Edit Printer Driver Configuration -> Base Properties TAB -> Advanced ->
Enable Merging of Background Raster = False
After you have changed the resolution go to File -> save; File -> exit.
The Print Dialogbox is now open go to File -> Reload printer Driver Configuration
Information from MicroStation help
Used to control whether multiple background rasters are merged together into a single composite background raster or written independently into the print output file. Background rasters are raster attachments that are printed below all non-raster graphics. It is more efficient to merge rasters, especially if the attachments overlap. However, Adobe Reader's Layers control cannot individually turn on or off rasters that have been merged inside PDF files. It may also be desirable to prevent merging of background rasters when producing Windows metafiles with transparent backgrounds. The default behavior is to merge background rasters. This property only applies when printing in non-rasterized mode.