Simplified code to show what the issue was

gistfile1.rb

# broked
def render_pdf(html, name)
  
  in_file   = "#{name}.xhtml"
  out_file  = "#{name}.pdf"
  
  # if the file name contains accents, they are changed to '??'
  File.open(in_file, 'w') do |f|  
    f.write html
  end
  
  # call java to build the PDF
  # this fails because the file written doesn't have the same name as we send java
  system("java generate_pdf #{in_file} #{out_file}")
  
  send_file out_file      
end

# fixed
def render_pdf(html, name)

  tmp_name  = Digest::SHA1.hexdigest(name)
  in_file   = "#{tmp_name}.xhtml"
  out_file  = "#{tmp_name}.pdf"
  
  # this now saves the name with the hash instead (no accents guaranteed)
  File.open(in_file, 'w') do |f|  
    f.write html
  end
  
  # so this now works
  system("java generate_pdf #{in_file} #{out_file}")
  
  # and we set the filename so the end user sees the right thing
  send_file(out_file, {
    :filename => "#{name}.pdf"
  })   
end