Prohibit the editing
To PDF files, you can have a variety of security measures, one of which is set not change the document setting.
Once you have set up a "master password" when creating PDF files, to "Do not allow changes in the document", unless you clear the "Do not allow changes in the document" Enter the password, etc. Adobe Acrobat, using the Acrobat you can not edit it.
As well as the prohibition of editing, you can also browse ban or a ban on printing.
If you fear that important documents like the password would have been leaking like editing, for some reason is a little annoying, you can also use the electronic signature feature.