What is your doubt exactly? Do you want to save the offer letter of the employee to master data? For this you can go to PA30 > Enter personnel number > In the screen where Maintain HR Master Data is written in bold, you will find an icon for Services for Objects. Click there and you can save the attachment.
Please make sure that you can create employee master data with or without this attachment. For Creation of employee, you do the hiring through PA40, selecting the appropriate action type. You cannot create any attachment while performing any action.
Hope this clarifies your doubt.
Regards,
Bhagyashree
