It was added at 10.1. It is the last parameter and is called show_selection_symbology.
exportToPDF (out_pdf, {page_range_type}, {page_range_string}, {multiple_files}, {resolution}, {image_quality}, {colorspace}, {compress_vectors}, {image_compression}, {picture_symbol}, {convert_markers}, {embed_fonts}, {layers_attributes}, {georef_info}, {jpeg_compression_quality}, {show_selection_symbology})
Jeff