The files need to be separate i.e the data in Excel the documents in Word or Publisher (Word will provide more functionalty - Publisher makes the production of stylish documents simpler). The documents can be connected seamlessly to the data.
If you can reproduce the certificate design in Word I would do so. You can then either use a VBA userform to read the data into a combo box or list box and select the record(s) to be processed, or use mail merge, but as Paul says, they will be separate documents.
See
http://www.gmayor.com/Userform.htm and for a more in depth explanation, see
http://gregmaxey.com/Create_and_employ_a_UserForm.htm