My one site's traffic has dropped in single day from around 4000 to 800 visitors. My other sites are getting normal visitor count.I'm not using any blackhat techniques nor trading links or having a bad neighbourhood or any copied materies. But yes it's a pdf documents download site with bounce rate of 32% and 3.5 pageviews per person.No issues reported in Google webmastertools. I find that for the relevant keywords my site has been pushed far way in Google.What could be the reason?EDITI've uploaded two updated reports( webmaster top keywords and analytics 1 month graph). 
What is the reason for this site's sudden drop in traffic?
traffic;google ranking