I contacted the creator of the WickedPDF gem, Miles Sterrett, and he recommended the following, which worked perfectly!
attachments["#{@project.title}.pdf"] = WickedPdf.new.pdf_from_string(render_to_string(:pdf => "MindMap", :template => 'projects/show.pdf.html.erb', :locals => {:project => @project, :user => @user }), {:orientation => 'Landscape'})
Big thanks to Miles for responding promptly, and for building such a useful gem!