Format isn't as important as the content. Make sure all of the following is included and that should be enough.
It should be an official document printed on company letterhead
Include the applicant's name, the company's contact information [address, telephone number and e-mail address]
Include the name, title and signature of the immediate supervisor or personnel officer at the company
Indicate all positions held while employed at the company
Include the following details: job title, duties and responsibilities, job status (if current job), dates worked for the company, number of work hours per week and annual salary plus benefits.