here is the list of supported data formats... Supported raster dataset file formats—Help | ArcGIS Desktop
If you desired import format is not directly supported, you may be able to export to a format that is and is on the list
You could convert the PDF to Raster dataset PDF To TIFF.
If the PDF has georeference information, the Raster can also fetch the same georeference information from it (Georeference information would be essential if you want to create a mosaic of multiple Rasters created from the PDF).