scatterplot of 100000 points and pdf file format

Hi,

I want to draw a scatter plot with 1M  and more points and save it as pdf.
This makes the pdf file large.
So i tried to save the file first as png and than convert it to pdf.
This looks OK if printed but if viewed e.g. with acrobat as document
figure the quality is bad.

Anyone knows a way to reduce the size but keep the quality?

I would strongly suggest a different method to present the data such as a 
contour plot or 3D bar plot.  An XY plot with a million points is unlikely to 
be readable unless it is produced as a large format print.  At 200 DPI 
printed, 1,000,000 discrete points requires a minimum of a 5 inch (12.7          
cm) by 5 inch area.  Besides, other than being visually overwhelming, what 
information would such a plot offer a viewer?

John