pdf file changes

I am an employer that uses a payroll program for direct deposit for my company payroll.  The program creates a pdf payroll stub to send weekly to my employees.  The program automatically send the stubs to my outlook.com email.  When I look at the stub in outlook, before I send them out, they are fine, but after I send them out and receive them, the pdf is totally different and unreadable.  I have tried changing the email format to plain text or HTML and the results are the same.  Any ideas on why the pdf changes when sent through outlook.