How to set page orientation for Word document?

Answer is right.

I just had to add extra dependencies to be able to access CTPageSz class.

// SBT config
"org.apache.poi" % "poi-ooxml" % "4.1.0",     // Base library
"org.apache.poi" % "ooxml-schemas" % "1.4",   // required to access CTPageSz

You were very much on the right path. Setting the orientation to landscape describes the general orientation of the paper, but will still need the size of the paper. Your CTPageSz object doesn't have that yet.

This means in addition to your setOrient call, you'll need to both setW and setH. These calls take BigIntegers that are representative of 1/20 Point. For a landscaped LETTER type paper therefore, you'll just:

pageSize.setW(BigInteger.valueOf(15840));
pageSize.setH(BigInteger.valueOf(12240));

For Word to recognize it as Landscaped, the width must be greater than the height. You still want to keep the setOrient call as well if you want it to behave properly when going to print.

Here's some common paper sizes in points from https://www.gnu.org/software/gv/manual/html_node/Paper-Keywords-and-paper-size-in-points.html you should take these and multiply them by twenty to use in the above method calls

Letter       612x792
LetterSmall  612x792
Tabloid      792x1224
Ledger       1224x792
Legal        612x1008
Statement    396x612
Executive    540x720
A0           2384x3371
A1           1685x2384
A2           1190x1684
A3           842x1190
A4           595x842
A4Small      595x842
A5           420x595
B4           729x1032
B5           516x729
Folio        612x936
Quarto       610x780
10x14        720x1008