WickedPdf doesn't alias_method_chain :render_to_string in ActionMailer like it does for ActionController.
First, update wicked_pdf to version 0.8.0 or git master. This will allow you to include it on an actionmailer class:
Then you get around this by manually including the PdfHelper module and just calling the method directly like so:
# Mailer
class Notifications < ActionMailer::Base
include PdfHelper
def send_email
archivo = render_to_string_with_wicked_pdf(
:pdf => "formulario_de_registro.pdf",
:footer => { :html => { :template => 'layouts/pdf/footer.pdf.haml' } }
# etc...
)
attachments['formulario_de_registro.pdf'] = archivo
mail :to => 'person@example.com'
end
end