There is an assumption in your initial post that pdf files cannot be modified by a third party. Unless a higher level of security has been set when the pdf file was generated, it is a relatively simple matter to read it into a word processor or DTP program, change the content and re-issue it under the same (pdf file) name. In this respect an ordinary pdf file is no more secure than any other file type.
If my memory serves me right - and I stress that I have only rarely used any of these features - the security settings include
:password protection, to prevent unauthorised viewing of the content, :printing lock, to stop paper copies being made, :content lock, to ensure that sections of the content cannot be copied or changed, :master password, to prevent the security settings from being over-ridden.
If any of these are important to you, the simplest solution might be to switch to a modern DTP program with pdf output and security settings built in. This needn't be expensive - the later versions of Serif PagePlus, for example, contain these features and may be obtained for £10-£40.
I think the Adobe website contains the specs of the pdf standard.