128 - 256, a practical difference ?

If you're not using an Open password, there's not much practical difference. If a PDF can be opened, there is software available to instantly remove any restrictions you have placed. PDF consumers (programs that read PDFs) are expected to honor any restrictions that have been placed with standard security.

If you've included an Open password, it would be difficult for someone who did not know the password to open the file (assuming a strong password). Much more difficult with 256-bit encryption than 128-bit. If you used a public key to encrypt for certain users it would be difficult for someone without the corresponding private key(s) to open the PDF. But if the Open password is known, any restrictions can be removed (or ignored), assuming standard security.

You're right about your digital signature not being relevant. It is used to verify the integrity of the file and that you are the one who applied the signature, not to enforce any restrictions.

There are DRM (Digital Rights Management) solutions for PDFs that you might want to look into.

George

OK, I appreciate your reply. Now let me get this straight, because my knowledge is very limited.

If I use a secure password for either Open Password or Change Permission Password, it would be difficult for someone to remove restrictions. It would be more difficult with 256 than 128.

The question remains is whether it's worth it to use 256 knowing than only the 9.0 Reader will read the PDF, or whether 128 is really enough and 7.0-9.0 Readers will be able to read the PDF? I know this is a judgment call, but what do you think?

I'm assuming there's really no difference in security whether you use the Open Password or the Change Permissions Password. Is that correct?

My purpose is to let anyone read it but allow no one to change it or print it.

Thanks.

If a PDF can be opened, restrictions can be easily removed, but not with Acrobat (unless the Change password is known). Other software is required, but it is readily available. Using an Open password doesn't change this. An Open password prevents someone who does not know the password from opening it.

DRM approaches might be able to give you what you want. Using standard security will not.

It really boils down to this: If you absolutely don't want anyone to be able to print or change your PDF, then don't make it available. If you want to prevent MOST people from printing or changing your PDF, then standard security is adequate.

George

If you use "password" open protection in Acrobat make sure you take into consideration the older versions of Reader or Acrobat that your end users will be using.