What exactly is it the password for?
Is this for PDF or something else. If it is for something else, you're in the wrong forum. If this is for PDF then is this for already encrypted PDFs or with for encrypted PDFs? Does this happen with all PDFs or only with some? Does this happen with PDFs that you were able to open in the past but not anymore?
It is possible that you misspelled your password on some PDFs and now do not remember that.