Only applicable if you know the password** and want to remove password because entering password, again and again, is tiring.* Just drag and drop your pdf file here: https://smallpdf.com/unlock-pdf
Also, passwords are put for a reason to safeguard the file so always removing the password is not good options multiple times keeping the password will be better but not always
If you don't the password then you are left with brute force method where it might take too much of time if the password is a general word and if it's alphanumeric with a symbol then not worth doing.