https://qiita.com/yancya/items/2992cd21cf390570f47a

pdf_firstpage.rb

require 'origami'

pdf = Origami::PDF.read(ARGV[0], verbosity: Origami::Parser::VERBOSE_QUIET)
unless pdf.pages.first.Resources.Font.C2_0
  exit 1
end

font_table = {}
pdf.pages.first.Resources.each_font do |name, font|
  puts name if $DEBUG
  utbl = font_table[name.to_s] = {}
  font.ToUnicode.data.each_line do |line|
    if line =~ /\A<([^>]+)>\s*<([^>]+)>(?:\s*<([^>]+)>)?/
      if $3
        puts line if $DEBUG
        base = $3.hex
        start = $1.hex
        start.upto($2.hex) do |i|
          utbl[sprintf('%04X', i)] = sprintf('%04X', base + (i - start))
          puts "#{sprintf('%04X', i)} => #{sprintf('%04X', base + (i - start))}" if $DEBUG
        end
      else
        utbl[$1.upcase] = $2
        puts "#{$1} => #{$2}" if $DEBUG
      end
    end
  end
end
puts font_table.keys.inspect if $DEBUG

utbl = font_table.values[0]
sentence = ''
line = pdf.pages.first.Contents.data
while /(?:\/(C2_\d+)\s[^<]+)?<([0-9A-F]+)>/m.match(line)
  rest = $'
  if $1
    puts $1 if $DEBUG
    utbl = font_table[$1]
  end
  sentence << $2.split('').each_slice(4).map(&:join).map do |w|
    x = utbl[w]
    unless x
      puts "#{w} not found" if $DEBUG
      x = '0000'
    end
    x
  end.map(&:hex).pack('U*')
  line = rest
end
puts sentence.strip