Yes, it is quite acceptable. Thumb impression has to be attested by two good witnesses, preferably third parties. Your father’s identity can be established by means of his PAN Card or Voter’s ID or Aadhaar Card.In the case of an illiterate or severely disabled / incapacitated testator, he may execute the Power of Attorney by a thumb mark – if male his left hand thumb impression and if female her right hand thumb impression shall suffice to execute the document.It is necessary that both the witnesses must sign in the presence of the testator.
The agreement should be typed/printed on one side in black ink on 60 GSM preferably on ledger paper. It is better to pay the proper stamp duty on or before its execution. To execute the agreement, sub-registrar require to put photographs, signature and left hand thumb impressions of all concern parties to execution.
In case of a male LEFT THUMB IMPRESSION shall be taken.
Since illiterates, blinds, and other disabled persons cannot put their signature they express their consent by placing thumb impression upon the document for various purposes like execution of loan documents, affidavits, declarations, making a will, opening bank account, withdrawing money from their account, etc.
Under Section 73 of the Indian Evidence Act the Court can direct any person to write any words or figures and to give his thumb impression for the purpose of enabling it to compare the words or figures so written or the impressions so taken with any words or figures or impressions alleged to have been written or affixed by such person.
Reference: Indian Evidence Act