You can save your Gmail messages in Google Docs and then edit them like any other document. The other use case is that once a Gmail message is available inside Google Docs, you can easily export that email as a PDF file or as a Word document while retaining the original formatting and without requiring an external PDF writer.
To integrated Google Docs with Gmail, go to the Labs page and enable the “Create a Document” gadget. Save the changes and return to your Gmail Inbox.
Next, open any of your existing email messages and you should see a “Create a Document” link under the More box as shown in the screenshot above. This will export the current message as a new document in Google Docs – the rich text formatting is preserved but if the message includes any file attachments, Google Docs will ignore them.
One more thing. The Google Docs app for Android allows offline access to your documents so once you have saved your most important Gmail messages in Google Docs, they can be accessed on your phone without a data connection as well.