How to avoid implementation dependent appearances of annotations and interactive form fields

Annotations such as notes and links belong to the most appreciated PDF features. Although they are easy to use from a user's perspective this is certainly not true from a developers perspective since it is often not clear how to render them.

Is the PDF version information a neglectable detail?

Each PDF file starts with a header comment that carries version numbers. What do they mean? Can a reader ignore these numbers or what is it to do? Is there any other version information that a reader should take care of?

Can linearization be combined with digitial signatures and PDF/A conformance?

Linearization is a feature to optimize PDF files for sequential reading. Although it is very useful in web based applications it interferes with other features such as digital signatures.