Can I tell the PI who has offered me a postdoc job that I have not decided yet waiting for other results I have got a postdoc job offer from a reputed University in Applied Mathematics. Now, I have given several postdoc interviews and some interviews are scheduled in the coming days. In such situation can I tell the PI (wherefrom I got the offer) that I have not decided yet as I have some other interviews scheduled in the future. I have heard that accepting multiple offers is not a good thing.
I think you are right about not accepting multiple offers. But that isn't the same as not considering multiple offers. 

Unless you have a special relationship with the PI you don't actually owe him or her an explanation of why you are waiting. It would, however, do you good if you could give a date, not too far in the future, by which you will let him/her know. 

Of course, it is possible that the offer could be withdrawn, rules permitting, but if you wait to accept you can't control that. 

I wouldn't probably say I was awaiting other offers, or give a false reason instead. It is enough to say that you aren't yet quite ready to make a decision on such a big step. Yes, that sort of thing is a bit of hedging, but probably called for, since you are actually hedging. 

But don't put them off too long.