ActsAsPdf

gem 'acts_as_pdf'

class Bill < ApplicationRecord

  acts_as_pdf :field, {method: :generate_bill, font_size: '10', font_name: 'Arial', line_height: '180%'}

  def generate_bill user, company, obj # has many arguments or no one 
    {
      variables: {
        "//COMPANY_NAME//": company.company_name,
        "//COMPANY_FULL_ADDRESS//": company.company_full_address,
        "//COMPANY_USER_POST//": company.company_user_post,
        "//USER_FIRST_NAME//": user.first_name,
        "//USER_LAST_NAME//": user.last_name,
        "//OBJECT_PRICE//": obj.salary,
        "//OBJECT_CURRENCY//": obj.currency
      },
      options: { # optional, only if you need footer or header
        header: { font_size: 16, center: 'header' }
        footer: { font_size: 10, center: 'footer'}
      }
    }
  end

end

class BillsController < ApplicationController
  preview_pdf
  # preview_pdf {view: 'preview.pdf'}
end

@pdf

= simple_form_for resource do |f|
    = f.text :field # with the markdown gem you want (be aware of whitespaces)
    = f.submit 'preview', name: 'preview',  action: 'preview', formtarget: "_blank", id: "first"
    = f.submit class: 'btn btn-custom inline', id: "second"

:javascript # in other file
  $('#first').removeAttr('data-disable-with')
  $('#second').removeAttr('data-disable-with')

    resources :bills do
      post :create, on: :member, action: :preview
      patch :update, on: :member, action: :preview
    end

class UsersController < ApplicationController

  def after_buying_something
    bill = Bill.find(param[:bill_id])
    product = Product.find(param[:product_id])
    @pdf = bill.generate_pdf(params, [@user, product.company, product])
  end
end

@pdf