It has been more than a year since I went on that account, I just realized that I can't remember the password of important pdf document

Talk to them (the PDF tech support people). They will have enough of a record, that they can help you in concert with what you already know. Your case is not unusual.

Wow! This is the 6th post in the last two hours wanting password help! Big memory loss out there! Passwords are meant to protect. If the folder has files important enough to password-protect, a note of it should have been made somewhere.