The Employers/firms misuse the policy of keeping certificates and bonds so that when the employee wants to quit for a better position, they will have to pay a good amount of money to get their certificates released.
Please read the clauses in the bond or exit conditions, notice periods and money to be paid if you have to resign from the job.
If you look at the information in the net, this plight is faced by many sectors. Luckily the court has always ruled in the favor of the employee and not the employer
For students (https://www.thehindu.com/news/cities/Madurai/colleges-cannot-withhold-students-certificates-hc/article4304170.ece)
What you can do is
1. Bottomline is, it is not legal or mandatory but for the sake of the job, you can give it to them.
2. If you have other offers in hand, then tell the first BPO that you will take the job as long as they accept attested copies of your certificates.
3. As long as you understand that these originals are a bargaining chip for the MNC to squeeze some money if you quit the job, then you are fine. If need be and when you quit you may have to negotiate for a 'fine' to get your certificates released.
4. Lastly, keep copies of these certificates. If you get a very good offer and the BPO is not willing to give you the certificates, try a simple settlement first, if not, then you can issue a lawyer notice and get your documents.
5. Practically, many people declare that they have lost the certificates during an interview travel, file an FIR, publish a small advert in the papers and then get a whole new set, although tedious, it can be done. So, I would say, go for it, the job market is tough, get the job, do not antagonize your prospective employers, if you reach a point of changing jobs, plan well, before you tell everyone.