I can not remember my password for signing documents with the digial signature.

You cannot recover the password. Cretae a new Digital ID and remember the password. If you import your Digital ID to the Acrobat's Digital IDs and not in Windows Certificate Store, you can select how often Acrobat will ask you for the password. Go to Preferences->Signatures->Identities&Trusted Certificates->More... Select 'Digital ID Files' and your certificate file in the left pane. Click 'Password Timeout' tab and it will give you choices. Do not select 'Never' as it will effectively remove password protection from your certificate unless you are the only one who has access to this computer. Otherwise, other people will be able to sign on this machine with your certificate pretending to be you.