If it is already a PDF, then wicked_pdf won't help you here, but you might be able to use Prawn or PDFtk.
You can layer some content on top of the bottom area of each page of an existing pdf with the following:
require 'prawn'
require 'pdf-reader'
respond_to do |format|
format.pdf {
input_filename = Rails.root.join('input.pdf')
page_count = PDF::Reader.new(input_filename).page_count
file = Prawn::Document.new(:skip_page_creation => true) do |pdf|
page_count.times do |num|
pdf.start_new_page(:template => input_filename, :template_page => num+1)
pdf.move_down(700)
pdf.text('FOOTER TEXT')
end
end
send_data file.render
}
end