Large PDF files when using tiff raster images as background

Dear list,

When plotting spatial informaion it is common to use a pixel map as 
background. Therefore I used the following commands (in brief)

library(sp)
library(rgdal)

pdf()
readGDAL(geotiff-file)
image()
plot(...,add=T)
dev.off()

This works fine, however, the original tiff file has a size of about 
600kb, the pdf 20MB even I do not add  any further information. Since I 
like to add point and line information with the plot command I would 
like to generate some kind of vecotrbased graphics. Any idea how I can 
the geotiff-background without producing large files?

Thanks for any hint in advance.