If you use LibreOffice and you are having problem to print in landscape mode, then you are not alone. Some people reported this on LQ and i myself also experienced this problem.
Some members reported a fix for this problem, but requires a lot of changes in the printer behaviour settings. Other member suggested that upgrading CUPS to 1.5.0 should fix this problem and this is what i choose.
Of course, it's not that smooth to upgrade to the latest cups. I had some problem building it and it was discussed on LQ, but finally i'm able to build cups by removing the old patches on Slackware-Current's SlackBuild script.
Now i'm able to build CUPS and fix the problem on LibreOffice. Hopefully this helps for those having same problems with me and with other Slackware users.