Dear Ajitha,
Did you get the answer to your query? Or, are you more confused? I would like to clarify in a more holistic way because you have not given any information about your company. But the fact that this question arose in your mind (here I agree with AnonymousA's comments that an HR executive is supposed to know these basics) implies that yours is a small company with no well-laid-out HR policies.
In traditional big Indian companies, PSUs, etc.: practice good people management practices. Every employee is considered/treated equal in terms of principles of employment. So whether he is a peon/messenger/office-boy - if he is in a regular role - he is entitled to all official documents related to his employment, (like any other employee), such as appointment letter, increment letter, promotion letter, appraisals, etc.
Midsized new-age companies (>20K Cr.), MNCs, etc.: Here, the trend is not to inflate headcount (as it affects productivity figures), so such and other similar services are outsourced to the extent possible, to a contractor. The system is exactly what Mr. Anonymous has described:
"all the office boys are on the payroll of our housekeeping contractor, and we pay a fixed amount to the housekeeping contractor for providing all services related to housekeeping. So we enjoy services of office boys, without any liability, so tomorrow if we don't want any office boy we can ask the contractor to remove them."
In such a scenario, of course, there is no question of an offer letter, etc.
Small companies: Here they follow an ad-hoc system. The peon/office boys dealing with important tasks - like those attached to Chairman, Directors, going to Govt. offices, Banks, etc. are on a regular roll; the rest are casual, temporary, etc. The regular office boys can be given an offer letter/appointment letter, etc.
But since these companies do not have any fair HR policies nor are very employee-friendly, it would be better that instead of seeking our opinion; you should ask your HR Director or your 'Honourable' Chairman for guidance or orders; else you may find yourself in trouble with the management.
Do contact me in case of any complications or further information.
Best regards.