after searching & testing several hours i reached the following situation:
It's seem like there is no single extension allowing an easy export of all articles to a single pdf - while there are several ways to generate pdfs from a collection of articles or single categories.
Extensions i did consider:
Extension: DumpHTML - working - but due to missing search (html) its not really useable as offside backup
Extension: PDF Export -> broken (see extension discussion/talk page)
Extension: Collection: complex config & requires online access or additional servers - might be a problem for specific setups.
Extension: PDF Book (pdf & html) - works pretty good for single categories - but is missing an option to select ALL articles.
A simple way to generate easy readable offsite backups of entire mediawiki installations - in best case the pdf-generation could be triggered from mediawiki itself but as well from cli (i.e. for cron-jobs/script usage)
I guess it could make sense to tweak Extension:PDFbook - as in theory/right now it should just miss an option to collect all articles. The pdf-generation in general is working already for single categories.
If that function would be added it would be pretty easy to generate pdf-backups from entire wikis - which might help several people overall.