Can I delay a while signing an official postdoc offer letter I have got an official postdoc offer letter. The HR now is requesting my signature and then return the letter back asap. But I haven't made a final decision to sign it or not, because there may be another offer coming very soon, which seems better to me. I wonder if it is appropriate to delay the received offer a little bit. After that accept or turn it down.
Sitting on an offer (e.g. ignoring emails from HR, taking forever to answer or other delay tactics) is generally not appropriate in my opinion. I would suggest honesty and transparency. 

Let your prospective postdoc advisor that you'd like a bit of time to consider your options (they will likely get the hint that you're considering other positions, no need to be too blunt). In addition, talk to your alternative option ASAP and let them know the situation. It is absolutely acceptable to tell them that you have another offer who's waiting for your reply, and you'd appreciate an answer from them soon. 

Whether either party would agree really depends on them. I would be careful of losing both options: if you wait for too long then your potential host might get annoyed and just rescind the offer. They could be holding off on other good applicants thinking that you are about to sign, and you waffling might result in them going ahead with other applicants. 

Your other option might not be getting back to you because they actually don't have an open position right now, or they themselves are waiting for some other applicant's final answer (if they say yes, you're out). 

These are all considerations that you should weigh out for yourself, and I cannot offer any concrete advice on. Just to reiterate - lying/misrepresenting your status/using delay tactics is probably a bad idea.