There is a 32 character maximum password length accepted in PDF versions 1.6 and older. You can type in more characters when setting the password using many pdf programs but all pdf readers will only consider the first 32 characters of a password for these pdf formats and will grant access if these are correct.
For pdfs secured in Adobe 9 and above, although the encryption is AES256, the password handling procedure is weaker than pdfs created using Abobe 7 or before, making passwords more susceptible to brute force attacks. For example, instead of a typical machine being able to test about 50,000 passwords per second for password protected pdf files created using Abobe 7 or before, (PDF formats 1.3, 1.4, 1.5 &1.6), that same machine might be able to test up to 100 million passwords per second for PDF formats 1.7 and above. This means that shorter passwords 8 characters or less are not secure under this format. It should be emphasized that it is the password that is more vulnerable and not the AES encryption key. If you go ahead and use PDF format 1.7 or above to secure a document, you need to choose a password that is at least 5 characters longer than you would if you used PDF format 1.6, to get the same level of security.
When encrypting a pdf file with a password, two options are usually offered. The first is setting an open/user password. This is a password you must enter before you can even open the file. The second is called an owner/master password which is a password you need to enter after you have opened the pdf document to enable you to do certain things to the document such as to print or edit it. It is critically important that users understand that only the open or user password is secure when encrypted with AES126, and when the password is sufficiently complex enough. The owner/master password on the other hand is easily broken with widely available password cracking programs on the internet and is not secure in any form whatsoever.
To get the greatest level of security, when adding a password to a pdf file, you should only use the PDF encryption software that is recommended by this website and only use the version number recommended by this website. The software we recommend is most likely to be the one that will have the most real world testing experience in the future from its many users who choose to secure their pdf documents in this way. If there are any failings, then you will hear about it.