Use imagemagik and Capture2Text to perform OCR

png2txt.rb

im = "C:\\Program Files\\ImageMagick-7.0.7-Q16\\magick.exe"
c2t = "H:\\Users\\Giovanni\\Downloads\\Capture2Text_v4.5.1_64bit\\Capture2Text_CLI.exe"

Dir.mkdir('res') unless Dir.exists?('res')

require 'thread/pool'
pool = Thread.pool(8)

Dir['*.png'].each do |f|
  pool.process {
    `"#{im}" convert \"#{f}\" -transparent white "res/#{f}"`
    `"#{c2t}" --vertical -l Japanese -i "res/#{f}" -b --trim-capture --deskew -o "res/#{f}.txt"`
  }
end

pool.shutdown

require "prawn"

Prawn::Document.generate("output.pdf") do
  font "mona.ttf"

  Dir['res/*.txt'].each do |f|
    text File.open(f, "r:UTF-8", &:read)
    start_new_page
  end
end