I don't think it's possible right now without coding.

If you are not afraid of coding, what you could do is something like :

1. add you membership certificate after the content of the receipt (in templates) and add an arbitrary separator between your receipt and the certificate. In my case <!--break-->.

2. use the hook_civicrm_alterMailParams with the following content (extract the part after <!--break--> and put it in a pdf as a attachment.

   $pos = strpos($params['html'], '<!--break-->');
   if ($pos !== False) {
     $pdf_content = substr($params['html'], $pos + 12);
     $params['html'] = substr($params['html'], 0, $pos);
   
     if (empty($params['attachments'])) {
       $params['attachments'] = array();
     }
     $params['attachments'][] = CRM_Utils_Mail::appendPDF(ts('Membership certificate') . '.pdf', $pdf_content);
   }
   

I may create a little extension for this.

I don't think it's possible right now without coding.

If you are not afraid of coding, what you could do is something like :

1. add you membership certificate after the content of the receipt (in templates) and add an arbitrary separator between your receipt and the certificate. In my case <!--break-->.

2. use the hook_civicrm_alterMailParams with the following content (extract the part after <!--break--> and put it in a pdf as a attachment) :

   $pos = strpos($params['html'], '<!--break-->');
   if ($pos !== False) {
     $pdf_content = substr($params['html'], $pos + 12);
     $params['html'] = substr($params['html'], 0, $pos);
   
     if (empty($params['attachments'])) {
       $params['attachments'] = array();
     }
     $params['attachments'][] = CRM_Utils_Mail::appendPDF(ts('Membership certificate') . '.pdf', $pdf_content);
   }
   

I may create a little extension for this.