Rails, option_for_select used to download pdf with ajax and send_file() not working

Question

Quick background: Want to send a pdf to the client for download, some of the pdf's are static, and some we generate with pdftk.

So I hacked this together after looking at this question:

Rails 3 - how to send_file in response of a remote form in rails?

HTML

<select id="download_policy_docs_select">
  <%= options_for_select(@file_list) %>
</select>
<input id="download_policy_doc" type="submit" class="btn btn-info btn-180px" value="DOWNLOAD" />

Ajax

$('#download_policy_doc').click(function(){
    var doc = $('#download_policy_docs_select').val();
    var policy_name = $('h1 :first').html();
    console.log("hello");
    $.ajax({
        type: "POST",
        url: "/policies/ajax_get_policy_document.pdf",
        timeout: 3000,
        success: do_the_success,
        data: {document : doc, policy_name : policy_name}
    });
});

Policies Controller:

def policy_declaration(id)
  policy = Policy.find(params[:id] || id)
  return unless enforce_policy_access(policy)

  file_path = PolicyPdf.generate_policy_declaration(policy)

  send_file(file_path, :filename => "#{policy.name}_declaration.pdf", :disposition => 'Content', :type => 'application/pdf')
end

def ajax_get_policy_document
  policy = Policy.find_by_name(params[:policy_name])
  if params[:document] == 'proof_of_insurance'
   redirect_to :action => 'policy_declaration'
  elsif params[:document] == 'master_cert'
    #redirect_to PolicyPdf.get_policy_pdf_filepath(policy)
  elsif params[:document] ==  'im_declaration'
    redirect_to :action => 'policy_declaration'
  elsif params[:document] == 'im_master_cert'
   # redirect_to PolicyPdf.get_policy_pdf_filepath(policy)
  elsif params[:document] == 'invoice'
    redirect_to policy_invoice
  elsif params[:document] == 'legacy_cert'
    redirect_to :action => 'policy_declaration'
  else
    "it's broke!"
  end
end

My understanding was that while ajax can't download a file, if I redirected to this other method policy_declaration and let it send the file, with the correct :disposition then I could in fact download a file, because it was being sent from the server - but not as the response to the ajax request.

So, my questions are:

• Where did my understanding go wrong?
• Can I fix my current code to work as desired?
• If I can't do the above what should I do?

Even if the answer is 'no you can't do this use a form' I would still like to know where my understanding went wrong.

Also in case anyone asks why I tried to do it this way. I'm not very good with the rails form_helpers (which is what a lot of other questions said to do)... So if that's the answer, some help in that direction would be very appreciated.