If you did not enter a password, it should not be password protected. However, assuming that it is try entering nothing as the password and see if it will open. If not nothing then try a space. If that fails you should probably just go to a backup of your pdf file or regenerate it from the original documents.
There are 3rd party password crackers available on the net, but if and when you purchase a cracker, you are giving your professional cracker your personal information. Doesn't seem to be the best of ideas.
I think i might have the same issue as MymicFSO.
The issue is when you try to go to secure>manage security policies by default there would be 2 entries
Encrypt with certificate
Encrypt with Password option
For the second if i check on the policy details both user password and owner password are set to "Not Required"
So if I try to secure a document and choose the "Encrypt a password" option, since there is no default password setup then it will prompt me to enter what ever password I choose.
Now on another computer someone changed the setting and chose to put in a password for the "User password" under the "Encrypt a password" policy. So now whenever I secure a document using that machine using "Encrypt a Password" it uses the one already stored and does not prompt me to enter a new password.
Now it can still be changed by by to the password security settings manually but I wonder if it is possible to get back that option for it to just prompt me whenever i try to secure a document.