I forgot my security password

PDF encryptionGenerally, PDF files are with a password called User Password, which is used to open the file and view the document. To encrypt a PDF password does not simply mean you can’t open and view the document, but PDF files have five to seven encryption ways to protect the file for office needs. Owner password, the other password of PDF files, is another crucial PDF password, which is to protect the file from maybe being copied (to Clipboard), printed, or modified, etc. So, in speaking of PDF password, it doesn’t necessary mean the one which you use to open the file.
PDF encryption largely relies on the sophisticated algorithm that hardly to be reversed. For example, you are easy to calculate that 1000 plus 1 is 1001, however, the reverse is seemed impossible, for 1001 can be an answer of countless calculations. PDF encryption works in the similar way. In this sense, given a password, it’s easy to deduce and decipher some data; however, the reverse PDF password recovery seems impossible.
Generally speaking PDF encryption with 128 bits key value is much more secure than that of 32 bits. PDF encryption is accomplished by a key value, either 128 or 32 bits in size. When using the key, all the bytes are changed (unless they are decoded back with the same key) into meaningless codes, and the file is impossible to be opened. Only the meaningless codes are calculated back using the correct value is it possible to get the correct password and open the file. 128 bits has a lot more possible values than the one with 32 bits, so it is much more secure. If your password is forgotten and intend to recover PDF password, PDF Password Unlocker software could be faster, for it supports 128 bits encryption, either 128 bits RC4 or 128 bits AES encryption.

PDF files are composed with objects, like image Objects, graphic Objects, and text Objects, etc. The key is calculated for each PDF Object in the PDF file using local data, which is much hard to crack, as the key keeps changing.
Set and remove PDF passwordsAdd a PDF Password1. Click the Secure button in the Tasks toolbar, and choose Password Encrypt.
2. Click “Yes” to the prompt about changing the security, and if necessary, type the Permissions password that lets you change security settings. If you don’t know the password, contact the author of the PDF.
3. In the Password Security - Settings dialog box, set the security options as desired, confirm the password, click OK, and then click OK again.
4. Save the document to apply the security settings.
Note: You can also restrict editing capabilities when you certify a document, or when you apply a policy to a PDF.
Remove PDF passwords and security settingsYou can remove PDF passwords and security policies from an open PDF if you have the permissions to do so. If the PDF is secured with a security policy that resides on a server, the changes can be made only by the author of the policy or by a server administrator.
1. Do one of the following: Click the Secure button in the Tasks toolbar, and choose Remove Security.
Type your password, and click Yes to the prompt about removing the security.
In the Security tab of the Document Properties dialog box, choose No Security from the Security Method menu.
2. When prompted, specify the Permissions password, and then click OK. Recover PDF password in case you forgot it.Every coin has two sides. Even the PDF document is strongly encrypted with passwords, it is impossible to insure that it is secure. Once you forgot PDF password, the data in the encrypted PDF file are on the risk of unavailability. Unless fortunately the password knocks your memory again, you got the data lost or you have to give up the document eventually. With the ever-growing of strong encryption algorithms, decryption algorithms gradually come into existing, too. Though the cracking is far more difficulty than ever before, it is still of help to those expecting to recover PDF password and access the password protected PDF file. Take PDF Password Unlocker, a popular PDF password recovery software, for example, it has adopted the advanced decryption algorithms to crack 40-bit RC4, 128-bit RC4, 128-bit AES, and 256-bit AES encrypted passwords with 3 attack methods, Dictionary attack, Brute-force attack, and Brute-force with Mask attack
Reference: http://www.passwordunlocker.com/knowledge/pdf-encryption-and-pdf-password-recovery.html