[SalesForce] How to attach a pdf in salesforce email,edit the email body and the changes will reflect in atached pdf

There is already a nice blogpost on how to do this

http://corycowgill.blogspot.in/2013/12/capturing-signatures-with-html5-canvas.html

global with sharing class AnyObjectSignatureController {
   public AnyObjectSignatureController(){
   }

 @RemoteAction
 global static String saveSignature(String signatureBody, String parentId) {
   try{
    system.debug('Record Id == ' + parentId);
    system.debug(signatureBody);
    Attachment a = new Attachment();
    a.ParentId = parentId;
    a.ContentType = 'image/png';
    a.Name = 'Signature Capture.png';
    insert a;
    return '{success:true, attachId:' + a.Id + '}';
    }catch(Exception e){
        return JSON.serialize(e);
   }
   return null;
 }

}

Script

canvas = document.getElementById("signatureCanvas");
 context = canvas.getContext("2d");
 drawingUtil = new DrawingUtil(canvas);

 function saveSignature(){
  var strDataURI = canvas.toDataURL();
// alert(strDataURI);
strDataURI = strDataURI.replace(/^data:image\/(png|jpg);base64,/, "");
 //alert(strDataURI);
AnyObjectSignatureController.saveSignature(strDataURI,accountId,processResult);
}

Visualforce Page

<canvas id="signatureCanvas" height="200px" width="300px"/>
  <input id="saveSigButton" type="button" name="SigCap" onclick="saveSignature();"   value="Capture Signature"></input>

You can't get the PDF content unfortunately.

While Email Services do run under a 'Context User', you will notice that UserInfo.getSessionId() returns null from any InboundEmailHandler implementation you may have.

The HTML with JavaScript is serving up a client-side redirect to a Salesforce login page. This is because pdf.getContent() runs in a different execution context that also lacks a Session ID at the time of making the request to render the page.

Salesforce have documented this:

This method can't be used in _[...] Apex email services