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?


/E

Dipl. bio-chem. Witold Eryk Wolski
MPI-Moleculare Genetic
Ihnestrasse 63-73 14195 Berlin
tel: 0049-30-83875219                 __("<    _
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      wolski at molgen.mpg.de