I would prefer "PDF::Puppy::Database::Abstract". But as valdez mentioned, another option is to prefix 'Base', e.g. Base::PDF::Puppy::Database. In hindsight, I'd probably prefer that (or even prefixed Abstract::).

However, there's no real convention here. The important thing is that it makes sense to you, it's fairly clear to anyone who might be maintaining your code in the future, and that you're consistent throughout your codebase.