A lot of people think a site has a good alexa rank just do to the fact it gets a lot of visitors but in reality is because those users have the alexa toolbar installed.
A site that receives 3,000 visitors a day and 2,000 of those visitors have the alexa toolbar installed will have a better alexa rank than a site that gets 10,000 Visitors a day and only 500 of those visitors have the alexa toolbar installed.
for PR just get backlinks from sites with good PR