I have found one solution that seems to be a private hack / branch of the official tree of dw2pdf that works for getting output in landscape format. You can find the solution here:
https://github.com/samwilson/dokuwiki-plugin-dw2pdf/tree/issue-19
To have a page or a part of it being printed in landscape format you need to write above the page/section you want to be printed in landscape the codeline:
~~PDF:LANDSCAPE~~
and from that point on a pagebrake will apply after that print is in landscape format.
To revert back to Portrait output you need to enter in the page the code line:
~~PDF:PORTRAIT~~
Update:
The developer of the plug-in upgrade wrote me that it is best to use his samwilson branch. He merged the issue-19 hack into that and regularly keeps that branch up-to-date with the official dw2pdf release.
https://github.com/samwilson/dokuwiki-plugin-dw2pdf/archive/samwilson.zip
I hope that will help also others with the "landscape problem"
Good luck
Olaf