It is not necessary that if your PR is increase than your site ranking is also increases. If you want to increase site SERP for keywords than you need to generate good quality backlink for your site.
Web site PR and search engine ranking are not related with each other. If you have low PR site and if you work hard on that site than also you can get good SERP for your site. And same vice-versa. If you have high PR site and if you do not work on that site than you can not get SERP for your site. You have to do lot's of work for your site.
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