gem install pdfkit
sudo apt-get install wkhtmltopdf
which wkhtmltopdf
cp /usr/local/rvm/gems/ruby-2.0.0-p0/bin/wkhtmltopdf /usr/bin/
chmod a+x /usr/bin/wkhtmltopdf

You could test this in IRB:

require 'pdfkit'
kit_obj  = PDFKit.new("<html><body>Testing</body></html>", :page_size=>'Letter')
pdf_obj  = kit_obj.to_pdf
pdf_file = kit_obj.to_file('/tmp/testing.pdf')