There are times when you get a document via email as attachment in gmail, In such cases you may want to edit these documents right away but you may not have MS Office installed on the computer or there might be a case where you are reading the email on a public computer where you don’t want to download the document.
Gmail allows you to open and edit the document online with google docs
When ever you receive a document in gmail, you will see a link view to open the document besides the link to download the file as shown in the image below.
Just click the "View" link at the bottom of a Gmail message shown with the doc attachment and the viewer will take it from there. If you decide you want to edit the file, clicking "Edit online" will open it in Google Docs, or you can download it to your desktop from there.