You can purchase the full Adobe Acrobat software from
www.adobe.com (retail price approx $199 US) The latest version includes the ability to have digital signatures (likely what you are looking for) and the ability to lock files. There was a russian programmer who found a workaround to the passwording in Acrobat. I recommend stronger encryption if you are serious about your files security.
There are many free alternatives for creating PDFs; most of the free software has limited features or cannot do all the things you are wanting to do.
If you are still interested, do a search for Ghostscript or Print2PDF.
As for encryption, PGP (Pretty Good Privacy) is a good option for encryption of your data. If both users have MacOS9 you can use the "Finder encrypting" option of the MacOS.