Hello Satyajeet,
Yes, you are eligible to do CA course though you have studied PCM in your 12th but you should gain a little knowledge about commerce which will help you. No doubt it is one of the most difficult course and teaches you a lot. Once you clear it you are treated as elite . And… If people know that you have been a science student, you still get that incredulous look and limelight doesn’t end.

To cut it short, PCM does matter. It gives you a better chance to be a CA. It gives you a better understanding to become a CA. It helps you with a wider perspective to not only be a CA, but be successful in whatever career path you choose.

GOOD LUCK.