custom paperclip processor - lives in lib/paperclip_processors

merge.rb

module Paperclip
  class Merge < Processor
    def initialize file, options = {}, attachment = nil
      super
      @format = File.extname(@file.path)
      @basename = File.basename(@file.path, @format)
      @files = attachment.instance.files
    end

    def make
      src = @file
      dst = Tempfile.new([@basename, @format])
      dst.binmode

      file_paths = @files.map { |file| "#{Rails.root}/tmp/#{file.original_filename}" }

      all_page_file_paths = ""

      file_paths.each do |file_path|
        pdf = PDF::Reader.new(file_path)
        count = pdf.page_count
        counter = 0

        count.times do
          all_page_file_paths += "#{file_path}[#{counter}] "
          counter += 1
        end
      end

       Paperclip.run(
         'convert',
         "-density 150 #{all_page_file_paths} #{File.expand_path(dst.path)}"
        )

      dst
    end
  end
end