There are two ways to protect a pdf file, the first method is asking password to open a pdf file and the second method is asking password to permit editing and printing of the document.
For the first method, there is no way to open the file but for the second method, we can recommend this way to delete the password if you forgot the password.
you should remember, you can only use this method if you had not restricted printing the file.
1- Open your pdf file.
2- This file is protected.
3- Open your file with Google chrome.
4- Print your file.
5- Save your file as pdf and then click on save key.
6- Open your file you can see all restrictions had vanished.