Yes you should.
If you have got the offer letter that means you are hired. Mini companies value the skills of the candidates other than the academic marks. Even though the eligibility criteria is 70%, maybe it doesn't matter that you have 68% marks.
Candidates get an offer letter after clearing all rounds of interviews. But an appointment letter gets issued to a candidate only after they accept the offer letter. The offer letter indicates that the candidate has got the job only when both the parties have signed the agreement. Unless and until you get a offer letter on letter head of new company you cannot consider that you have been offered a job.  If you have received above letter from the company than yes you can take the job.
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