Come on already. Every single app out there supports PDF. How difficult is this? The whole paper usage while printing is a total red herring.
Topic on Talk:Reading/Web/PDF Functionality/Flow
Looking at the last paragraph the update on 15 of July it is very clear that there is currently no development going on in rendering books in any downloadable format, in particular including PDF. Furthermore it is also stated there, that no such development is currently planned.
If you refer to mediawikil2atex (one hour ≈ 200 pages in PDF) with your comment on paper usage, I can assure you that the limit is necessary since the hardware resources allocated by WMF together with the current implementation of mediawiki2latex don't reasonably allow for converting significantly bigger documents.
Still the downloadable command line version of mediawkiki2latex does not include any such limits, and should be able to fulfil everyone needs in terms of PDF size on a reasonably priced personal computer made for consumer use.